feat(files): a Files section over the caller's whole space

Until now file browsing existed only inside a project, and the two memory
stores were reachable only by the agent's tools. `#files` is the general
surface: home, both memory stores, the shared folders and projects the
caller belongs to, plus the read-only skills and docs trees.

The root is virtual, and that is the design. Anchoring at `~` is wrong:
the explorer reads host-side, while `shared/`, `projects/`, `skills/` and
`docs/` are bind mounts inside the container — a page rooted at the home
would show less than the user has with no way to reach the rest, and on
native Linux would show Docker's empty mountpoint stubs, a door that
appears to work and leads nowhere. So level 0 is a synthetic list from the
new `GET /api/files/roots`, serialized from the caller's `UserFs` plus the
two virtual memory roots. It sends `kind`, never a label: labels are copy
and get translated.

`GET /api/files/dir` now answers `{ path, can_write, entries }`, and a
memory path is classified before `resolve_view_path` (which refuses one)
and listed from `memory_docs`: one level derived from the flat key space
by the pure `memory_docs::immediate_children`, over a single query whose
unslashed prefix also spots an exact note as "not a directory". Memory is
read-only from the page — every writer routes through `resolve_view_path`,
and `shared-memory/*` is `@fs_write require` for the agent, so a button
that walks past that rule is a decision of its own.

The explorer moves out of projects into `shared/file-explorer.js`, taking
`root` + `rootLabel` and reading `can_write` from the listing rather than
from its host: writability changes per branch and comes from the same
`UserFs::can_write_to` the server rejects writes with, so the buttons
offered and the writes accepted cannot disagree. Deep-linking needed it
steerable without a two-way binding, hence `rel` in and
`explorer-navigate` out — the event fires only for a click, never for a
`rel` the host set, so echoing it back is a no-op.

The URL carries the agent path of the open folder in one parameter, the
same vocabulary the assistant uses, so a link is shareable and pasteable
into a conversation; which root it belongs to is derived, not stored.

docs/: a new files.md, plus two pages this made false — shared-folders.md
claimed in three places that a shared folder has no explorer, and
memory.md never said a user can now read their own notes.

docs/index.md

@@ -4,7 +4,7 @@ This folder is written for **you, the assistant**, not for the human directly. I
 
 Keep answers grounded in what's actually enabled and configured for this instance — check with the relevant tool (e.g. list installed/enabled plugins) rather than assuming everything described here is turned on. A feature documented here may not be enabled on this particular instance.
 
-This index will grow over time. Right now it covers the interface, agents, memory, projects, shared folders, background tasks, system agents, access grants, connectors, skills, the sandbox, voice input and plugins; more sections (security groups…) will be added later.
+This index will grow over time. Right now it covers the interface, files, agents, memory, projects, shared folders, background tasks, system agents, access grants, connectors, skills, the sandbox, voice input and plugins; more sections (security groups…) will be added later.
 
 ## Features
 
@@ -12,6 +12,7 @@ This index will grow over time. Right now it covers the interface, agents, memor
 | --- | --- |
 | [memory.md](memory.md) | Private and shared memory: what goes where, the indexes and history log, why some shared facts can't be changed on request |
 | [agents.md](agents.md) | Agents: the three kinds (chat, task, system), which one you are talking to and why, the specialist agents the assistant delegates to, how the model is chosen, and adding a custom agent |
+| [files.md](files.md) | The Files page: everywhere a member can reach — home, both memory stores, shared folders, projects, skills and docs — what they can change there, and what is deliberately read-only |
 | [projects.md](projects.md) | Projects: shared folders with their own assistant chat, a live file explorer, and member sharing |
 | [shared-folders.md](shared-folders.md) | Shared folders: admin-managed folders with no chat of their own — who sees them, read vs write access, why the assistant asks before touching them, and when to choose a project instead |
 | [system-agents.md](system-agents.md) | Background agents that run on a schedule (event triage, the two memory lints, the nightly conversation review of a supervised account): what they watch, why they only ever report, why a run can be skipped, and their settings |