fix(tmux): wait for shell ready before sending keys (#264)

Add WaitForShellReady call before SendKeys in all agent managers
(deacon, mayor, witness, refinery). This prevents intermittent
"can't find pane" errors that occur when the tmux session is
created but the shell isn't ready to receive input yet.

The issue manifests under load (e.g., during `gt up` when multiple
agents start in sequence) where the 200ms delay in SendKeysDelayed
isn't sufficient for the pane to be fully initialized.

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

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

internal/polecat/session_manager.go

@@ -183,6 +183,11 @@ func (m *SessionManager) Start(polecat string, opts SessionStartOptions) error {
 	if command == "" {
 		command = config.BuildPolecatStartupCommand(m.rig.Name, polecat, m.rig.Path, "")
 	}
+	// Wait for shell to be ready before sending keys (prevents "can't find pane" under load)
+	if err := m.tmux.WaitForShellReady(sessionID, 5*time.Second); err != nil {
+		_ = m.tmux.KillSession(sessionID)
+		return fmt.Errorf("waiting for shell: %w", err)
+	}
 	if err := m.tmux.SendKeys(sessionID, command); err != nil {
 		return fmt.Errorf("sending command: %w", err)
 	}