Migrate startup paths from SessionBeacon to StartupNudge (gt-7pp3l)

All 12 startup paths now use session.StartupNudge instead of the
older SessionBeacon pattern. The key difference is that StartupNudge
includes sender information, enabling better predecessor discovery
via gt seance.

Files updated:
- crew_lifecycle.go: 3 locations (refresh, restart)
- start.go: 2 locations (crew start)
- deacon.go: 1 location (deacon start)
- witness.go: 1 location (witness start)
- up.go: 4 locations (mayor, witness, crew, polecat)
- mayor.go: 1 location (mayor start)
- session/manager.go: 1 location (polecat spawn)

Format change:
Old: [GAS TOWN] address • molID • timestamp
New: [GAS TOWN] recipient <- sender • timestamp • topic[:mol-id]

This completes the Agent Startup Consolidation epic (gt-85whr).

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

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

internal/cmd/start.go

@@ -799,12 +799,13 @@ func runStartCrew(cmd *cobra.Command, args []string) error {
 		// Give Claude time to initialize after process starts
 		time.Sleep(constants.ShutdownNotifyDelay)
 
-		// Inject session beacon for predecessor discovery via /resume
+		// Inject startup nudge for predecessor discovery via /resume
 		address := fmt.Sprintf("%s/crew/%s", rigName, name)
-		beacon := session.SessionBeacon(address, "")
-		if err := t.NudgeSession(sessionID, beacon); err != nil {
-			// Non-fatal: session works without beacon
-		}
+		_ = session.StartupNudge(t, sessionID, session.StartupNudgeConfig{
+			Recipient: address,
+			Sender:    "human",
+			Topic:     "cold-start",
+		}) // Non-fatal: session works without nudge
 
 		// Send gt prime to initialize context
 		if err := t.NudgeSession(sessionID, "gt prime"); err != nil {
@@ -940,10 +941,13 @@ func startCrewMember(rigName, crewName, townRoot string) error {
 	// Give Claude time to initialize
 	time.Sleep(constants.ShutdownNotifyDelay)
 
-	// Inject session beacon for predecessor discovery via /resume
+	// Inject startup nudge for predecessor discovery via /resume
 	address := fmt.Sprintf("%s/crew/%s", rigName, crewName)
-	beacon := session.SessionBeacon(address, "")
-	_ = t.NudgeSession(sessionID, beacon) // Non-fatal
+	_ = session.StartupNudge(t, sessionID, session.StartupNudgeConfig{
+		Recipient: address,
+		Sender:    "human",
+		Topic:     "cold-start",
+	}) // Non-fatal
 
 	// Send gt prime to initialize context (non-fatal: session works without priming)
 	_ = t.NudgeSession(sessionID, "gt prime")