containers: drive user provisioning and remounts from the system bus

The endpoints that changed a user or a membership row also reached into
ContainerManager themselves: users_mgmt called ensure()/remove(), and both
shared_folders and projects called refresh_user_mounts through a local
remount() helper. Every future endpoint that grants membership would have
had to remember to do the same.

Announce instead. SystemEventBus gains UserCreated / UserDeleted /
UserMountsChanged, emitted after the DB write, and one subscriber —
wiring::spawn_user_lifecycle — does the Docker work: sequentially (which
serialises concurrent operations on the same container), best-effort by
contract (the row is already committed, so a hiccup settles at the user's
next login or at boot reconciliation), and holding only a Weak<Skald>. It
is spawned after construction, like set_skald, because it reacts through
Skald's own accessors.

Also fixes a real gap the event makes impossible to repeat: the web setup
wizard created the first admin without provisioning a container. It runs
against a live server, where reconcile_all() has already happened, so that
admin had no sandbox until the next restart. It now emits UserCreated like
any other creator; the console shell needs no equivalent, since it runs
before the server and boot reconciliation covers it.

Two behaviour changes: POST /api/users and POST /api/projects no longer
wait on Docker before responding. Provisioning was already best-effort, and
a new project's folder is still created synchronously, so the explorer —
which reads host-side — shows it at once; only execute_cmd reachability
lands a moment later.

CLAUDE.md

@@ -28,11 +28,13 @@ Three global broadcast buses — **never add a fourth without checking these fir
 | Bus | Cap | Events | File |
 |-----|-----|--------|------|
 | `ChatEventBus` | 256 | user message, assistant response, compaction done | `core-api/src/bus.rs` |
-| `SystemEventBus` | 64 | provider (un)registered, config key updated, job completed, session cancelled | `core-api/src/system_bus.rs` |
+| `SystemEventBus` | 64 | provider (un)registered, config key updated, job completed, session cancelled, **user created/deleted/mounts-changed** | `core-api/src/system_bus.rs` |
 | `GlobalEvent` (per-user) | 512 | all `ServerEvent` variants → WS clients + inbox lifecycle | `core-api/src/events.rs` |
 
 Plus internal `mpsc` queues: per-source `SourceInbox` (message serialization) and a central `notify` queue (background agents → user).
 
+**The user-lifecycle reconciler** is the worked example of the rule. Creating a user, deleting one, or changing a shared-folder/project membership all need Docker work (provision, tear down, recreate with new bind mounts). None of the endpoints that make those changes touches `ContainerManager`: each announces `SystemEvent::User{Created,Deleted,MountsChanged}` **after** its DB write, and one subscriber — `skald::wiring::spawn_user_lifecycle`, spawned post-construction because it reacts through `Skald`'s own accessors, holding only a `Weak` — does the reacting, sequentially and best-effort. The payoff is that a *future* endpoint granting membership cannot forget to remount, because remounting was never its job. Reactions never block the HTTP response, and a failure settles at the user's next login or at boot reconciliation.
+
 **Before you add a direct function call or a new import between two components, stop and ask:** is one component producing data another needs? If yes, add a variant to an existing bus and spawn a subscriber. Don't call `some_manager.log_thing(...)` from the producer — emit a `ThingHappened` event on `SystemEventBus` and let the manager subscribe.
 
 **A new `mpsc::channel` or `broadcast::channel` is a code-review flag.** Nine times out of ten you want one of the three buses above. If you truly need a new one, be ready to explain why none of the existing three fits.
@@ -158,11 +160,11 @@ Two views, **one storage**: the fs-tools run **host-side** in the Skald process
 
 **Containment** (`resolve_host_path`): every physical fs-tool op canonicalizes the resolved path (following symlinks) and prefix-checks it against its mount base, **fail-closed**. Since the same tree is writable from inside the container, a symlink planted there that points outside the home/shared root is caught here — the host-side tool never escapes the user's workspace. `grep_files` stays disk-only (regex ≠ FTS; memory → `memory_search`) but resolves its root the same way. `execute_cmd`'s `workdir` is an agent path mapped to its container path via `UserFs::to_container`.
 
-The threading: `UserContext.fs` (built by `container::build_user_fs` at login, snapshotting shared memberships) → `ChatSessionManager` → `ChatSessionHandler.fs` → `ToolContext.fs`. **Admin CRUD is wired** (`src/frontend/api/shared_folders.rs` — `GET/POST /api/shared-folders`, `PATCH/DELETE /api/shared-folders/{id}`, `POST`/`DELETE .../members[/{user_id}]`; UI `shared-folders.js`): a create/describe/delete + per-member `can_write` surface, and each mutation calls a best-effort `remount(user)` that rebuilds the affected user's fs + container mounts **in place** — so a membership change lands without a re-login (blueprint §6's "admin CRUD" + "membership refresh without re-login" TODOs, now closed; it still settles at next login/boot if the live remount fails). `execute_cmd` /stop is robust: the command runs under `setsid -w` in its own process-group (leader pid recorded in a container pidfile), and a `KillReaper` drop-guard reaps that group on /stop **or** timeout via a detached `docker exec` that walks `/proc` and kills members by **positive pid** (the container's dash mishandles `kill -<pgid>`); the pidfile is passed positionally (`$1`), and the container's `--init` (tini) reaps the killed processes so no zombies accumulate. **Per-user MCP connectors now run inside this container** (§7) — the container infra enabled it; see the MCP connectors section.
+The threading: `UserContext.fs` (built by `container::build_user_fs` at login, snapshotting shared memberships) → `ChatSessionManager` → `ChatSessionHandler.fs` → `ToolContext.fs`. **Admin CRUD is wired** (`src/frontend/api/shared_folders.rs` — `GET/POST /api/shared-folders`, `PATCH/DELETE /api/shared-folders/{id}`, `POST`/`DELETE .../members[/{user_id}]`; UI `shared-folders.js`): a create/describe/delete + per-member `can_write` surface, and each mutation emits `SystemEvent::UserMountsChanged`, on which the lifecycle reconciler runs `Skald::refresh_user_mounts` — rebuilding the affected user's fs + container mounts **in place**, so a membership change lands without a re-login (blueprint §6's "admin CRUD" + "membership refresh without re-login" TODOs, now closed; it still settles at next login/boot if the live remount fails). `execute_cmd` /stop is robust: the command runs under `setsid -w` in its own process-group (leader pid recorded in a container pidfile), and a `KillReaper` drop-guard reaps that group on /stop **or** timeout via a detached `docker exec` that walks `/proc` and kills members by **positive pid** (the container's dash mishandles `kill -<pgid>`); the pidfile is passed positionally (`$1`), and the container's `--init` (tini) reaps the killed processes so no zombies accumulate. **Per-user MCP connectors now run inside this container** (§7) — the container infra enabled it; see the MCP connectors section.
 
 ## Projects
 
-A **project** is a shareable, self-service workspace: a folder at `{WD}/projects/{owner_userid}/{slug}` plus membership in the registry. `projects` (accessor `db/projects.rs` — slug is immutable, `UNIQUE(owner_user_id, slug)`) + `project_members` (junction with `can_write`; the owner is always a write-member, so a private project = one member). Sharing is **not** admin-gated: the owner and any write-member can add/remove/re-grant members and edit metadata; only the owner can delete. Each membership mutation remounts the affected users' containers in place (`Skald::refresh_user_mounts`). The mount appears in the agent namespace as `projects/{owner_username}/{slug}` (host keys on the stable userid, agent path on the username) — read-only members get a read-only bind mount in the container.
+A **project** is a shareable, self-service workspace: a folder at `{WD}/projects/{owner_userid}/{slug}` plus membership in the registry. `projects` (accessor `db/projects.rs` — slug is immutable, `UNIQUE(owner_user_id, slug)`) + `project_members` (junction with `can_write`; the owner is always a write-member, so a private project = one member). Sharing is **not** admin-gated: the owner and any write-member can add/remove/re-grant members and edit metadata; only the owner can delete. Each membership mutation emits `SystemEvent::UserMountsChanged` for the affected user; the lifecycle reconciler remounts their container in place (`Skald::refresh_user_mounts`), so the folder is browsable at once (the explorer reads host-side) and reachable from `execute_cmd` a moment later. The mount appears in the agent namespace as `projects/{owner_username}/{slug}` (host keys on the stable userid, agent path on the username) — read-only members get a read-only bind mount in the container.
 
 **API** (`src/frontend/api/projects.rs`): `GET/POST /api/projects`, `GET/PUT/DELETE /api/projects/{id}`, `POST /api/projects/{id}/members`, `DELETE .../members/{user_id}`, `POST /api/projects/{id}/session`. `ProjectDetail` carries `root_path` — the agent path of the folder, computed server-side (owner username ≠ `owner_name`, which may be a display name) — the explorer's root. A `project-{id}` chat source provisions the `project-coordinator` agent with a project `RunContext` (`provisioning_for_source` → `skald_core::projects::build_project_run_context`: `project_root` + a system block with name/description/folder/members); every member keeps their **own private** `project-{id}` session — only the folder is shared.