Add session beacon for predecessor discovery via /resume

Inject an identity beacon as the first message when Gas Town starts Claude
sessions. This beacon becomes the session title in Claude Code '/resume
picker, enabling workers to find their predecessor sessions for debugging.

Beacon format: [GAS TOWN] <address> • <mol-id or "ready"> • <timestamp>

Examples:
- [GAS TOWN] gastown/crew/max • gt-abc12 • 2025-12-30T14:32
- [GAS TOWN] gastown/polecats/Toast • ready • 2025-12-30T09:15
- [GAS TOWN] deacon • patrol • 2025-12-30T08:00

Workers can now press / in /resume picker and search for their address
(e.g., "gastown/crew/max") to see all predecessor sessions.

Note: Respawn-loop agents (deacon/refinery via up.go) skip beacon injection
since Claude restarts multiple times - would need post-restart injection.

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

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

internal/session/manager.go

@@ -11,6 +11,7 @@ import (
 	"time"
 
 	"github.com/steveyegge/gastown/internal/claude"
+	"github.com/steveyegge/gastown/internal/constants"
 	"github.com/steveyegge/gastown/internal/rig"
 	"github.com/steveyegge/gastown/internal/tmux"
 )
@@ -185,6 +186,21 @@ func (m *Manager) Start(polecat string, opts StartOptions) error {
 		return fmt.Errorf("sending command: %w", err)
 	}
 
+	// Wait for Claude to start (non-fatal: session continues even if this times out)
+	if err := m.tmux.WaitForCommand(sessionID, constants.SupportedShells, constants.ClaudeStartTimeout); err != nil {
+		// Non-fatal warning - Claude might still start
+	}
+	time.Sleep(constants.ShutdownNotifyDelay)
+
+	// Inject session beacon for predecessor discovery via /resume
+	// This becomes the session title in Claude Code's session picker
+	address := fmt.Sprintf("%s/polecats/%s", m.rig.Name, polecat)
+	molID := opts.Issue // Use issue ID if provided
+	beacon := SessionBeacon(address, molID)
+	if err := m.tmux.NudgeSession(sessionID, beacon); err != nil {
+		// Non-fatal: session works without beacon
+	}
+
 	return nil
 }