Add timeout fallback for dead agents (gt-2hzl4)

- Add checkStaleAgents() to detect agents reporting "running" but not updating
- Add markAgentDead() to update agent bead state to "dead"
- Integrate stale agent check into heartbeat cycle
- DeadAgentTimeout set to 15 minutes

This is a safety mechanism for agents that crash without updating their state.
The daemon now marks them as dead so they can be restarted.

Also fixes duplicate AgentFields declaration - now uses beads.go version with
ParseAgentFieldsFromDescription alias in fields.go.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/daemon/daemon.go

@@ -185,6 +185,10 @@ func (d *Daemon) heartbeat(state *State) {
 	// 4. Process lifecycle requests
 	d.processLifecycleRequests()
 
+	// 5. Check for stale agents (timeout fallback - gt-2hzl4)
+	// Agents that report "running" but haven't updated in too long are marked dead
+	d.checkStaleAgents()
+
 	// Update state
 	state.LastHeartbeat = time.Now()
 	state.HeartbeatCount++