feat(prompt): tell the agent what its sandbox can run

The agent had no way to know its container ships ffmpeg, ripgrep or
tesseract, so it either declined work it could do or spent a round finding
out. This adds a command list to the system prompt as a **discovery hint** —
explicitly not an inventory.

Every decision follows from it being a hint:

- The allowlist (~35 entries, `container/commands.rs`) is the curation; a
  full PATH dump is 800 entries of coreutils noise. The probe exists so the
  list cannot *lie*, not so it can discover: `command -v` at login means we
  never announce something a container recreate threw away.
- The rendered prose says the list is partial and names `command -v`, so a
  tool outside the allowlist costs one check rather than a wrong conclusion.
  An empty probe renders as an explicit "could not be read", never as
  silence under a heading promising a list.
- Order is the allowlist's own, grouped by kind of work — the grouping is
  the curation, and the reader is a model, not a grep.
- Staleness is cheap both ways, so there is no invalidation machinery: a
  login-time snapshot on `UserContext`, non-fatal, refreshed at next login.

The gate is the tool, not the sentinel. Every AGENT.md carries
`common/sandbox.md` — the four system agents included — and the section is
emitted iff the turn's model is shown `execute_cmd`, derived from
`allow_tools` plus the security group's visibility filter for a root turn
and from `child_defs` for a sub-agent: always the same definitions the model
will see. `has_execute_cmd` therefore joins the PrefixCache key, since the
group is switchable mid-conversation and that switch already rewrites the
tool payload in the same provider cache.

The fragment holds only the heading and one stable sentence; every
conditional claim lives in the renderer, because prose promising
`sudo apt-get install` is not the renderer's to retract when the tool is
absent. `execute_cmd`'s own description loses `(python + node available)`:
its job is steering away from the shell, and a capability advertisement
diluted it.

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, voice input and plugins; more sections (security groups…) will be added later.
+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.
 
 ## Features
 
@@ -19,6 +19,7 @@ This index will grow over time. Right now it covers the interface, agents, memor
 | [settings.md](settings.md) | The admin's Config page: interface language, the compaction model picker, debug mode |
 | [access.md](access.md) | Who can use which plugin or connector: the open default, removing access per person, and the role switch that keeps children out of it |
 | [connectors.md](connectors.md) | Connectors (MCP servers): shared vs per-user, setting one up in the UI, the sign-in and QR-pairing flows, and what to do when one is not working |
+| [sandbox.md](sandbox.md) | Your sandbox: the private Linux container commands run in, which files survive a rebuild, why the command list in your prompt is partial, and installing what is missing |
 | [skills.md](skills.md) | Skills: instruction folders the assistant loads on demand — where they live, how to read and run one, and the contract for writing, installing and downloading one |
 | [voice.md](voice.md) | Voice input: configuring a transcription model, and why the microphone button does nothing unless the page is served over HTTPS or localhost |
 | [interface.md](interface.md) | The desktop interface: collapsing the sidebar to an icon-only strip to make room for documents |