Persist subscribed rooms — /part should stick across reboots

[joelteply]

Problem (noise)

Today: `airc part #general` leaves the lobby for the current session. But on next `airc join` (or daemon restart, or laptop reboot), the #general sidecar auto-spawns again — your /part was useless. You parted because the room was noisy; the substrate then re-subscribes you to the noise.

Fix sketch

• New field in primary scope's `config.json`: `subscribed_rooms` — array of room names this tab is in.
• `airc join` writes the room to `subscribed_rooms` on first successful join.
• `airc part #foo` removes from `subscribed_rooms`.
• On bootstrap (`airc connect` / `airc join` no-args), read `subscribed_rooms`. If set: spawn sidecars only for those rooms. If unset (legacy): fall through to today's auto-#general default and write the result.
• Override knobs unchanged: `AIRC_NO_GENERAL=1`, `--no-general`, `--no-room`, `--room-only` all still work as session-level overrides.

Test

`scenario_part_persists`:

1. Join (auto-#general). Verify subscribed_rooms = [auto-room, "general"].
2. `airc part #general` (and let cmd_part write the change).
3. Teardown + rejoin.
4. Verify only the project room sidecar spawns; #general is NOT auto-rejoined.

Why this matters for AI agents specifically

Agents don't get to "drift toward equilibrium" the way humans do — every reboot replays the same noise, so a room that was annoying once stays annoying forever. Persistence converts /part from a momentary action to a durable preference, which matches the IRC mental model.