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_test.go

@@ -11,11 +11,11 @@ func TestDefaultBackoffConfig(t *testing.T) {
 	if config.Strategy != StrategyGeometric {
 		t.Errorf("expected strategy Geometric, got %v", config.Strategy)
 	}
-	if config.BaseInterval != 60*time.Second {
-		t.Errorf("expected base interval 60s, got %v", config.BaseInterval)
+	if config.BaseInterval != 5*time.Minute {
+		t.Errorf("expected base interval 5m, got %v", config.BaseInterval)
 	}
-	if config.MaxInterval != 10*time.Minute {
-		t.Errorf("expected max interval 10m, got %v", config.MaxInterval)
+	if config.MaxInterval != 30*time.Minute {
+		t.Errorf("expected max interval 30m, got %v", config.MaxInterval)
 	}
 	if config.Factor != 1.5 {
 		t.Errorf("expected factor 1.5, got %v", config.Factor)
@@ -29,11 +29,11 @@ func TestNewAgentBackoff(t *testing.T) {
 	if ab.AgentID != "test-agent" {
 		t.Errorf("expected agent ID 'test-agent', got %s", ab.AgentID)
 	}
-	if ab.BaseInterval != 60*time.Second {
-		t.Errorf("expected base interval 60s, got %v", ab.BaseInterval)
+	if ab.BaseInterval != 5*time.Minute {
+		t.Errorf("expected base interval 5m, got %v", ab.BaseInterval)
 	}
-	if ab.CurrentInterval != 60*time.Second {
-		t.Errorf("expected current interval 60s, got %v", ab.CurrentInterval)
+	if ab.CurrentInterval != 5*time.Minute {
+		t.Errorf("expected current interval 5m, got %v", ab.CurrentInterval)
 	}
 	if ab.ConsecutiveMiss != 0 {
 		t.Errorf("expected consecutive miss 0, got %d", ab.ConsecutiveMiss)