feat(lanes): Antigravity lanes, release exited panes, fresh Codex starts

- antigravity: a third lane tool running Antigravity's `agy` CLI. `-i`
  carries the first prompt, `--continue` resumes; `agy` has no model,
  effort or permission flag, and `--dangerously-skip-permissions` is never
  emitted. No pipeline, no agents install, and no AI Router (native only,
  else EUNSUPPORTED). Add lane gains an Antigravity choice and the binary
  probe maps it to `agy`.
- UI: the lane card no longer renders a frame holding only a "Codex" chip,
  and the prompt box names the lane's CLI ("Ask Codex anything…"), via
  LANE_TOOL_LABEL.
- runs: a pane that exits is released on a 10 s server beat
  (healAllRunIds, DASHBOARD_RUN_HEAL_MS). Codex and Antigravity fire no
  hook, so their lanes stayed "running" until something re-read them.
- codex: Start no longer sends resumeLast — only "Resume last session"
  continues the previous thread.

server/README.md

@@ -679,7 +679,7 @@ Reads — and carefully gated mutations for low-risk text-file artifacts — for
 
 ### Run a lane's CLI (`/api/run`)
 
-HTTP surface for the tmux-backed terminal run: one session per lane, named `agenthub-lane-<laneId>`, running the CLI that lane declares (`claude` or `codex`). There is no in-memory handle map — every read is computed from `tmux list-sessions`, so a session killed out-of-band self-corrects on the next call. Every route enforces a same-origin / loopback-Origin guard against browser CSRF.
+HTTP surface for the tmux-backed terminal run: one session per lane, named `agenthub-lane-<laneId>`, running the CLI that lane declares (`claude`, `codex`, or `agy` for an `antigravity` lane). There is no in-memory handle map — every read is computed from `tmux list-sessions`, so a session killed out-of-band self-corrects on the next call. Every route enforces a same-origin / loopback-Origin guard against browser CSRF.
 
 | Method   | Path                          | Description |
 | -------- | ----------------------------- | ----------- |