Crew sessions: use respawn-pane for cleaner lifecycle (gt-09i4)

Replace SendKeys approach with respawn-pane -k when starting Claude
in crew sessions. This gives cleaner exit behavior:
- Before: Claude exits → shell prompt → exit shell → session ends
- After: Claude exits → session ends (no intermediate shell)

Changes:
- Add GetPaneID() to tmux package for pane ID retrieval
- Update crew_at.go to use RespawnPane for both new and restart cases
- Remove unnecessary waits and multi-step Claude startup

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

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

internal/tmux/tmux.go

@@ -236,6 +236,20 @@ func (t *Tmux) GetPaneCommand(session string) (string, error) {
 	return strings.TrimSpace(out), nil
 }
 
+// GetPaneID returns the pane identifier for a session's first pane.
+// Returns a pane ID like "%0" that can be used with RespawnPane.
+func (t *Tmux) GetPaneID(session string) (string, error) {
+	out, err := t.run("list-panes", "-t", session, "-F", "#{pane_id}")
+	if err != nil {
+		return "", err
+	}
+	lines := strings.Split(out, "\n")
+	if len(lines) == 0 || lines[0] == "" {
+		return "", fmt.Errorf("no panes found in session %s", session)
+	}
+	return lines[0], nil
+}
+
 // GetPaneWorkDir returns the current working directory of a pane.
 func (t *Tmux) GetPaneWorkDir(session string) (string, error) {
 	out, err := t.run("list-panes", "-t", session, "-F", "#{pane_current_path}")