fix(statusline): ensure crew sessions have correct hook display

Root cause: tmux statusline showed wrong hook for all java crewmembers
because GT_CREW env var wasn't set in tmux session environment.

Changes:
- statusline.go: Add early return in getHookedWork() when identity is empty
  to prevent returning ALL hooked beads regardless of assignee
- crew_at.go: Call SetEnvironment in the restart path so sessions created
  before GT_CREW was being set get it on restart

Fixes gt-zxnr.

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

internal/cmd/statusline.go

@@ -713,6 +713,12 @@ func getMailPreviewWithRoot(identity string, maxLen int, townRoot string) (int,
 // beadsDir should be the directory containing .beads (for rig-level) or
 // empty to use the town root (for town-level roles).
 func getHookedWork(identity string, maxLen int, beadsDir string) string {
+	// Guard: identity must be non-empty to filter by assignee.
+	// Without identity, the query would return ALL hooked beads regardless of assignee.
+	if identity == "" {
+		return ""
+	}
+
 	// If no beadsDir specified, use town root
 	if beadsDir == "" {
 		var err error