feat: Add C-b n/p keybindings for crew session cycling

When multiple crew sessions exist in the same rig, C-b n cycles to next
and C-b p cycles to previous. Sessions are sorted alphabetically and
wrap around.

Implementation:
- crew_cycle.go: Hidden `gt crew next/prev` commands for tmux to call
- crew_helpers.go: parseCrewSessionName and findRigCrewSessions helpers
- crew_at.go: Calls SetCrewCycleBindings on session creation

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

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

internal/cmd/crew_at.go

@@ -103,6 +103,9 @@ func runCrewAt(cmd *cobra.Command, args []string) error {
 		theme := getThemeForRig(r.Name)
 		_ = t.ConfigureGasTownSession(sessionID, theme, r.Name, name, "crew")
 
+		// Set up C-b n/p keybindings for crew session cycling
+		_ = t.SetCrewCycleBindings(sessionID)
+
 		// Wait for shell to be ready after session creation
 		if err := t.WaitForShellReady(sessionID, 5*time.Second); err != nil {
 			return fmt.Errorf("waiting for shell: %w", err)