refactor: ZFC cleanup - move Go heuristics to Deacon molecule (gt-gaxo)

Remove Go code that makes workflow decisions. All health checking,
staleness detection, nudging, and escalation belongs in the Deacon
molecule where Claude executes it.

Removed:
- internal/daemon/backoff.go (190 lines) - exponential backoff decisions
- internal/doctor/stale_check.go (284 lines) - staleness detection
- IsFresh/IsStale/IsVeryStale from keepalive.go
- pokeMayor, pokeWitnesses, pokeWitness from daemon.go
- Heartbeat staleness classification from pokeDeacon

Changed:
- Lifecycle parsing now uses structured body (JSON or simple text)
  instead of keyword matching on subject line
- Daemon now only ensures Deacon is running and sends simple heartbeats
- No backoff, no staleness classification, no decision-making

Total: ~800 lines removed from Go code

The Deacon molecule will handle all health checking, nudging, and
escalation. Go is now just a message router.

See gt-gaxo epic for full rationale.

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

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

internal/keepalive/keepalive.go

@@ -86,26 +86,7 @@ func Read(workspaceRoot string) *State {
 // Returns a very large duration if the state is nil.
 func (s *State) Age() time.Duration {
 	if s == nil {
-		return 24 * time.Hour * 365 // Very stale
+		return 24 * time.Hour * 365 // No keepalive
 	}
 	return time.Since(s.Timestamp)
 }
-
-// IsFresh returns true if the keepalive is less than 2 minutes old.
-func (s *State) IsFresh() bool {
-	return s != nil && s.Age() < 2*time.Minute
-}
-
-// IsStale returns true if the keepalive is 2-5 minutes old.
-func (s *State) IsStale() bool {
-	if s == nil {
-		return false
-	}
-	age := s.Age()
-	return age >= 2*time.Minute && age < 5*time.Minute
-}
-
-// IsVeryStale returns true if the keepalive is more than 5 minutes old.
-func (s *State) IsVeryStale() bool {
-	return s == nil || s.Age() >= 5*time.Minute
-}