feat: signpost the virtual memory roots inside the container, instead of leaving them absent

`user-memory/` and `shared-memory/` live in SQLite, so nothing of them existed on
disk — and that nothing was worse than it looks. `cat user-memory/x.md` returned a
bare ENOENT, which a model reads as "the note is missing" rather than "wrong door";
and `mkdir -p user-memory && echo … > user-memory/x.md` *succeeded*, writing a real
file into the home that no reader ever visits (every reader goes to `memory_docs`)
and that the next `ls` then confirms as if it had worked.

Each root now gets a read-only bind mount holding a README that names the tools to
use instead. Read-only as a mount rather than as a mode: the container user has
passwordless sudo, so a chmod would be a suggestion, while `:ro` holds — remounting
needs CAP_SYS_ADMIN. Verified in a scratch container: write, sudo write, sudo chmod,
sudo mount -o remount,rw and sudo rm all fail. And a README rather than an empty
directory, because "Permission denied" is an error, not an instruction — models
answer it by reaching for sudo; the README puts the correction in the same directory
the failing command just named.

The mounts are deliberately not part of `UserFs`: they back no agent path and the
host-side fs-tools must never resolve into them. They reach existing containers as a
fourth self-heal axis in `reusable()`, not as an IMAGE_TAG bump — the image is
unchanged, and a bump would make every installation rebuild it to fix a mount.

The matching half is in `classify_memory`, which now strips the home spellings
(`./`, `~/`, `/root/`) before matching the root. Without it `~/user-memory/x.md`
missed the match and fell through to the disk router — becoming exactly the
invisible physical file the signpost exists to prevent.

`agents/common/memory.md` says the rule outright: the stores are reachable only
through the file tools and `memory_search`, never through `execute_cmd`.

CLAUDE.md

@@ -166,6 +166,8 @@ The agent sees **one namespace**, routed on the first path component. The choke
 
 Two views, **one storage**: the fs-tools run **host-side** in the Skald process on `{WD}/homes/{userid}` + `{WD}/shared/{X}`; `execute_cmd` runs **inside the container** (`docker exec -w <container-path> skald-{userid} sh -c …`, via `ExecuteCmd::run_with`) on the same paths bind-mounted (`homes/{userid}`→`/root`, `shared/{X}`→`/root/shared/{X}`, read-only when `can_write=0`). A file written in the container appears to the host fs-tools and vice versa.
 
+**The memory roots are signposted inside the container, not merely absent.** `user-memory/`/`shared-memory/` are virtual, so nothing of them existed on disk — and the nothing was worse than it sounds: `cat user-memory/x.md` returned a bare ENOENT (which reads as *the note is missing*, not *wrong door*), while `mkdir -p user-memory && echo … > user-memory/x.md` **succeeded**, writing a real file into the home that no reader ever visits and that the next `ls` then confirms as if it had worked. Each root is therefore a **read-only bind mount** (`{WD}/.memory-signpost/{root}` → `{container_home}/{root}:ro`, gitignored, rewritten from consts on every `ensure`) holding a README that names the tools. Read-only *as a mount*, not as a mode: the container user has passwordless `sudo`, so a `chmod` would be a suggestion, whereas `:ro` holds — remounting needs `CAP_SYS_ADMIN` (verified: write, `sudo` write, `sudo chmod`, `sudo mount -o remount,rw` and `sudo rm` all fail). A README rather than an empty dir because `Permission denied` is an error, not an instruction — models answer it by reaching for `sudo`; the README puts the correction in the directory the failing command just named. These mounts are deliberately **not** in `UserFs`: they back no agent path and the host-side fs-tools must never resolve into them. They are the **fourth self-heal axis** in `reusable()` (`signposts_mounted`) rather than an `IMAGE_TAG` bump, since the image is unchanged and a bump would make every box rebuild it to fix a mount. The matching half is in `classify_memory`, which now strips the home spellings (`./`, `~/`, `/root/`) before matching the root — without it `~/user-memory/x.md` missed the match, fell through to the disk router, and became exactly the invisible physical file the signpost exists to prevent.
+
 **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 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.