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.

crates/skald-core/src/session/manager.rs

@@ -68,6 +68,9 @@ impl ChatSessionManager {
         max_parallel_subagents: usize,
         max_tool_result_chars: Option<usize>,
         datetime_config:       DatetimeConfig,
+        // The sandbox commands snapshotted at this user's login — see
+        // `crate::container::commands`.
+        sandbox_commands:      Arc<Vec<String>>,
         tools:                 Arc<ToolRegistry>,
         mcp:                   Arc<dyn McpProvider>,
         approval:              Arc<ApprovalManager>,
@@ -100,6 +103,7 @@ impl ChatSessionManager {
                 // configured message cap.
                 auto_compaction_enabled: compactor.auto_enabled(),
                 datetime:              datetime_config.clone(),
+                sandbox_commands,
                 max_agent_depth:       crate::session::handler::MAX_AGENT_DEPTH as u32,
             },
         )?;