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.

client/src/components/lanes/AddLaneModal.tsx

@@ -429,6 +429,15 @@ export function AddLaneModal({
               label={t("addLaneToolCodex")}
               onClick={() => setTool("codex")}
             />
+            <Seg
+              active={tool === "antigravity"}
+              label={t("addLaneToolAntigravity")}
+              onClick={() => {
+                setTool("antigravity");
+                // `agy` cannot run behind an AI Router (see router-apply.js).
+                setRouter("native");
+              }}
+            />
           </div>
         </div>
 
@@ -635,7 +644,10 @@ export function AddLaneModal({
             hid it. The pipeline picker is gone entirely: every lane is created
             on `default` (the server's own default) and the template is changed
             from the lane's own header afterwards. */}
-        <div className="rounded-md border border-border-light bg-surface-0 p-2">
+        <div
+          className="rounded-md border border-border-light bg-surface-0 p-2"
+          hidden={tool === "antigravity"}
+        >
           <div>
             <fieldset>
               <legend className="mb-1 text-xs text-fg-secondary">{t("addLaneRouterLabel")}</legend>