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/lib/lane-agents.js

@@ -66,8 +66,8 @@ async function commonGitDir(cwd) {
  * any existing copy — this is a reinstall, not a merge) and idempotently
  * git-exclude that directory in the shared common git dir.
  *
- * A `codex` lane gets nothing: Codex never reads `.claude/agents/`, and codex
- * lanes carry no pipeline to drive.
+ * A `codex` or `antigravity` lane gets nothing: neither CLI reads
+ * `.claude/agents/`, and those lanes carry no pipeline to drive.
  *
  * @param {{cwd: string, tool?: string}} lane
  * @returns {Promise<{installed: string[]}>}
@@ -79,7 +79,7 @@ async function installAgents(lane) {
     });
   }
 
-  if (lane.tool === "codex") return { installed: [] };
+  if (lane.tool && lane.tool !== "claude") return { installed: [] };
 
   const dest = path.join(lane.cwd, ".claude", "agents");
   fs.mkdirSync(dest, { recursive: true });