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/sandbox.md

@@ -0,0 +1,27 @@
+# Your sandbox
+
+Every member of this instance has their **own private Linux container**, and you work inside theirs. It is where `execute_cmd` runs, and it is separate from everyone else's: nothing you do in one person's sandbox is visible from another's.
+
+## What is in it
+
+Mounted into it are the places you already know by name: the home directory (`~`), the shared folders that person belongs to, their projects, and the read-only `skills/` and `docs/` trees. Everything else in the container — `/tmp`, `/etc`, an installed package's files — belongs to the sandbox alone.
+
+The distinction matters for one reason: **the mounted directories survive, the rest does not.** A container can be rebuilt at any time (a software update, a change to someone's folder access), and when it is, it comes back from a clean image. Files under `~`, the shared folders and the projects are untouched. Anything installed into the container is gone.
+
+## What you can run
+
+Your prompt lists **some** of the commands the sandbox provides — the common ones, checked at the start of the session so the list never claims something that is not there. It is a shortcut, not an inventory: the sandbox has far more than the list shows, and a command missing from it may well be installed. Check any specific one with `command -v <name>`.
+
+You are free to work in there as you see fit, including installing what you need:
+
+```
+sudo apt-get install -y <package>
+```
+
+No password is needed. Because an install is lost when the container is rebuilt, prefer installing quietly as part of doing the work over telling the user to install something — and if a task depends on a heavy tool being present every time, say so, so an admin can have it added to the base image.
+
+If a user asks what the assistant can *do* with files, media or documents, the honest answer is grounded here: a full Linux environment with the usual toolbelt, in which you can also install what is missing.
+
+## When you cannot run commands
+
+`execute_cmd` is not always available. A restrictive security group can withhold it, and some background agents are given no tools at all by design. When that happens your prompt says so plainly instead of listing commands — take it at face value and do the work with the tools you do have, or explain what you would need.