[Bug] taskGateways Map in room-store never cleaned up

## Summary

`packages/core/src/stores/room-store.ts` — the module-scoped `taskGateways` Map grows indefinitely. When a room is created via `initConductor()`, a `taskGateways.set(taskId, gatewayId)` entry is added, but it is never removed when the room is stopped, archived, or deleted.

## Expected behavior

Add cleanup logic in `setRoomStatus()` when status transitions to a terminal state (`stopped`, `error`), or provide an explicit `removeRoom()` action.

## Files

- `packages/core/src/stores/room-store.ts` — `taskGateways` Map (line ~66)

## Context

Introduced in PR #210.

Start in packages/core/src/stores/room-store.ts at the module-scoped taskGateways Map and inspect setRoomStatus() plus the room lifecycle paths. Ensure entries created by initConductor() are no longer retained after terminal room states or removal, and verify the Map is cleaned up for stopped and error cases.

Written by the indexing model from the issue text.