feat(updates): deliver app update notifications to Agent threads

[huntharo]

Problem

Electron update availability currently appears through the app update UI, but Agent-bound messaging conversations cannot receive update lifecycle notifications or safely request an upgrade from chat.

The Agent-thread messaging work made Agent threads useful as operational surfaces. Update availability should be another event-driven Agent notification source, not a timer automation and not renderer-toast-only state.

Proposal

Add event-driven update notifications that can be attached to selected Agent threads.

When an update is available or downloaded, PwrAgent should post a source-labeled update card to configured Agent threads. An authorized actor in an Agent-bound messaging conversation should be able to say upgrade.

On upgrade, PwrAgent should:

• inspect active and queued thread work before restarting
• ignore busy threads that are attached only to the initiating Agent
• block on unrelated busy threads
• maintain a sticky upgrade-status card listing blocking threads
• update the waiting list as threads start, finish, or queue
• install/restart when the unrelated busy-thread list reaches zero
• persist restart intent so startup can report completion or recovery back to the initiating Agent

Alternatives

Keep update notifications as renderer-only toast state. That is simpler, but it misses headless/messaging workflows and cannot coordinate a restart with active Agent-thread work.