feat(deacon): improve timing and add heartbeat command

Timing changes for more relaxed poke intervals:
- Daemon heartbeat: 60s → 5 minutes
- Backoff base: 60s → 5 minutes
- Backoff max: 10m → 30 minutes
- Fresh threshold: <2min → <5min
- Stale threshold: 2-5min → 5-15min
- Very stale threshold: >5min → >15min

New command:
- `gt deacon heartbeat [action]` - Touch heartbeat file easily

Template rewrite:
- Clearer wake/sleep model
- Documents wake sources (daemon poke, mail, timer callbacks)
- Simpler rounds with `gt deacon heartbeat` instead of bash echo
- Mentions plugins as optional maintenance tasks
- Explains timer callbacks pattern

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

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

internal/daemon/backoff.go

@@ -34,11 +34,14 @@ type BackoffConfig struct {
 }
 
 // DefaultBackoffConfig returns sensible defaults.
+// Base interval is 5 minutes since deacon rounds may take a while
+// (health checks, plugins, syncing clones, complex remediation).
+// Max interval is 30 minutes - beyond that, something is likely wrong.
 func DefaultBackoffConfig() *BackoffConfig {
 	return &BackoffConfig{
 		Strategy:     StrategyGeometric,
-		BaseInterval: 60 * time.Second,
-		MaxInterval:  10 * time.Minute,
+		BaseInterval: 5 * time.Minute,
+		MaxInterval:  30 * time.Minute,
 		Factor:       1.5,
 	}
 }