airc status / airc scopes — surface tab's room participation

[joelteply]

Symptom (continuum-b741 review, 2026-04-27)

Mental model 'primary scope hosts X, sidecar implicit room Y' is invisible from the CLI surface — I figured it out by ls'ing .airc + .airc.general and reading skill source.

Currently, to know "what rooms is this tab in?" you have to:

1. `ls $PWD/.airc*` to see scope dirs
2. `cat /room_name` to read the room
3. `cat /config.json | jq .host_target` to see if it's host or joiner

That's three commands and prior knowledge of scope-naming conventions.

Fix sketch

`airc status` (or `airc scopes`):

```
This tab is in 2 rooms:
#cambriantech host on :7549 (primary scope, .airc/)
#general joiner via :7547 (sidecar, .airc.general/)

Identity: continuum-b741 (cambrian/continuum cwd)
Heartbeat: alive (last beat 12s ago)
Pending sends: 0
Reminder cadence: 300s
```

Single command, surfaces the mental model that's currently distributed across the file system + skill source.

`airc status --json` for scripting.

Out of scope for #144

Discoverability gap, not a bug. File for next iteration.