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/daemon/daemon_test.go

@@ -206,32 +206,8 @@ func TestSaveLoadState_Roundtrip(t *testing.T) {
 	}
 }
 
-func TestIsWitnessSession(t *testing.T) {
-	tests := []struct {
-		name     string
-		expected bool
-	}{
-		{"gt-gastown-witness", true},
-		{"gt-myrig-witness", true},
-		{"gt-my-rig-name-witness", true},
-		{"gt-a-witness", true},  // minimum valid
-		{"gt-witness", false},   // no rig name
-		{"gastown-witness", false}, // missing gt- prefix
-		{"gt-gastown", false},      // missing -witness suffix
-		{"gt-mayor", false},        // not a witness
-		{"random-session", false},
-		{"", false},
-		{"gt-", false},
-		{"witness", false},
-	}
-
-	for _, tc := range tests {
-		result := isWitnessSession(tc.name)
-		if result != tc.expected {
-			t.Errorf("isWitnessSession(%q) = %v, expected %v", tc.name, result, tc.expected)
-		}
-	}
-}
+// NOTE: TestIsWitnessSession removed - isWitnessSession function was deleted
+// as part of ZFC cleanup (gt-gaxo). Witness poking is now Deacon's responsibility.
 
 func TestLifecycleAction_Constants(t *testing.T) {
 	// Verify constants have expected string values