Add vim list nav, fix window-title padding toggle, widen title spacing

TUI:
- VimListView: j/k now move down/up in the project and session lists
  (arrow keys still work); used by both list widgets.

tmux status bar:
- Widen the gap between window cells via a 2-space window-status-separator
  (WINDOW_STATUS_SEPARATOR).
- Fix the current-window cell toggling between padded " 1: name " and
  unpadded "1: name". The per-window status format is static, but
  set_window_label re-asserted it on every 3s poll — letting a competing
  writer (e.g. a stale TUI process running older code with a differently
  padded format) flip the cell each cycle. The format is now owned by a
  single write-once path (new_window at creation, apply_theme for existing
  windows); the poll updates only the dynamic @hqt_label.

Also lands pre-staged scaffolding: AGENTS.md quality gate, TODO.md backlog,
and ty dev dependency.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

TODO.md

@@ -0,0 +1,6 @@
+P1: One long-lived SQLAlchemy session is shared across Textual workers and the 3-second poller, creating concurrency and stale-state risk.
+P1: Missing session rows or stale harness rows can crash service methods instead of returning user-facing errors.
+P1: Database setup uses `Base.metadata.create_all` only, so existing user databases will not be migrated when schema changes.
+P2: Codex session-id capture is heuristic and can store the wrong rollout if another Codex session starts in the same project during the retry window.
+P2: tmux behavior is mostly tested with mocks; add isolated real-tmux smoke tests for theming, new windows, respawn, and labels.
+P2: Project paths are accepted without existence validation, so bad paths become dead sessions later instead of being rejected early.