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/router-apply.js

@@ -187,6 +187,13 @@ function applyToCodexLane(lane, profile) {
  */
 function applyRouterToLane(lane, profileName) {
   const profile = routers.getRouter(profileName); // throws ENOTFOUND
+  // `agy` has no endpoint/model override to write, so only native fits.
+  if (lane.tool === "antigravity") {
+    if (profile.builtin) return { tool: "antigravity", file: null, profile: profile.name };
+    throw Object.assign(new Error("an Antigravity lane cannot use an AI Router"), {
+      code: "EUNSUPPORTED",
+    });
+  }
   if (lane.tool === "codex") {
     // Native is the base config as it already stands. A layer that overrode
     // nothing would still shadow whatever model the user set in the base.