Board auto-selects a spawned room's channel via a focus marker

[rogeriochaves]

The board now snaps to the room you just spawned

When a room spawns, the gateway brings the KanbanCode board forward, but it did not auto-select the room's channel, so you still had to click into it to watch the agents self-organize. That channel auto-select was dropped earlier when we removed the kanbancode:// deep link (the deep link relaunched a stale registered build and tripped KanbanCode's quit-protection dialog mid-demo).

This restores it without a deep link. focusChannel now writes the channel slug to ~/.kanban-code/focus-channel before bringing the app forward. The KanbanCode app reads and drains that marker in applicationDidBecomeActive and selects the channel. A plain marker file needs no relaunch, so the quit dialog stays gone.

Contract (paired with the KanbanCode app-side watcher)

• File: ~/.kanban-code/focus-channel
• Content: the bare channel slug (room.channel, e.g. dark-mode), no #, no newline. The app-side read trims whitespace and strips a leading #, so either form is tolerated.
• The app drains (deletes) the marker on read, so each spawn writes a fresh one and nothing accumulates.
• Written before the System Events activate, so it is on disk when applicationDidBecomeActive fires.

Safety

Best-effort: the write is wrapped so a failure (missing dir, permissions) just means no auto-select, never a broken board-focus. The two halves are independent and harmless alone (the watcher no-ops with no marker; the marker no-ops with no reader).

Verification

• Drove focusChannel("marker-probe-xyz") headless: the marker file held exactly marker-probe-xyz (bare slug, no newline).
• Typecheck clean; gateway hot-reloads cleanly.
• Joint test with the rebuilt app: spawn a room, board snaps to its channel.