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/index.js

@@ -504,6 +504,21 @@ function startBackgroundServices() {
   } catch (err) {
     console.warn("codex poll failed to start:", err.message);
   }
+  // A codex/antigravity pane that exits fires no hook, so release its lane on
+  // a beat instead (one `tmux has-session` per lane with a run). Off with
+  // DASHBOARD_RUN_HEAL_MS=0.
+  const HEAL_MS = Number(process.env.DASHBOARD_RUN_HEAL_MS ?? 10_000);
+  if (Number.isFinite(HEAL_MS) && HEAL_MS > 0) {
+    const { healAllRunIds } = require("./routes/lanes");
+    const healTimer = setInterval(() => {
+      try {
+        healAllRunIds();
+      } catch (err) {
+        console.warn("run heal failed:", err.message);
+      }
+    }, HEAL_MS);
+    if (healTimer.unref) healTimer.unref();
+  }
   // Continuous discovery of sessions under ~/.claude/projects. The one-time
   // legacy backfill above runs only once (marker-gated), so a project added
   // later whose sessions never flow through hooks would otherwise stay invisible