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/__tests__/router-apply.test.js

@@ -233,6 +233,24 @@ describe("router-apply — the written file must survive a lane reset", () => {
   });
 });
 
+describe("router-apply — Antigravity lane", () => {
+  const agyLane = () => ({ id: 9, cwd: LANE_CWD, tool: "antigravity" });
+
+  it("accepts native and writes nothing", () => {
+    assert.equal(applyRouterToLane(agyLane(), "native").file, null);
+    assert.equal(fs.existsSync(SETTINGS), false);
+  });
+
+  it("refuses a real router instead of writing Claude's settings file", () => {
+    const { name } = saveNineRouter();
+    assert.throws(
+      () => applyRouterToLane(agyLane(), name),
+      (err) => err.code === "EUNSUPPORTED"
+    );
+    assert.equal(fs.existsSync(SETTINGS), false);
+  });
+});
+
 describe("router-apply — Codex lane", () => {
   it("writes a profile layer file, never the base config", () => {
     const base = path.join(process.env.CODEX_HOME, "config.toml");