fix(lanes): bridge routes/lanes.js to pty-run.js

Replace run-spawner imports and APIs with pty-run:
- Import pty-run instead of run-spawner
- Delete setRunExitHandler registration, replace with read-time self-heal in payload()
- Remove mode validation (mode no longer exists in pty-run)
- Update spawnRun call to use new parameter names (initialPrompt, not prompt/mode)
- Replace "message" action with explicit 400 EUNSUPPORTED response
- Fix stopLaneRun to poll on status !== "gone" instead of !actualExitedAt

Adapt tests to tmux-based run model:
- Delete tests about mode-specific behavior (removed feature)
- Rewrite lane release tests using tmux.__setExecImpl mocks instead of withFakeClaude
- Update assertions to check status === "gone" instead of specific exit codes
- Update ERUNTIMEOUT test to mock tmux sessions instead of child processes

All lane-related tests pass; only pre-existing port conflicts in lane-detect.test.js remain.

server/__tests__/lanes-api.test.js

@@ -645,14 +645,12 @@ describe("lane actions", () => {
     await request("DELETE", `/api/lanes/${c.body.lane.id}`);
   });
 
-  it("message on a lane with a recorded-but-not-live run returns 409", async () => {
+  it("message on a lane is no longer supported via REST", async () => {
     const c = await request("POST", "/api/lanes", { cwd: "/tmp/lane-action-e" });
     const id = c.body.lane.id;
-    // Patch the lane with a bogus run_id (never existed, so not live).
-    await request("PATCH", `/api/lanes/${id}`, { run_id: "nonexistent-run" });
     const r = await request("POST", `/api/lanes/${id}/message`, { text: "hello" });
-    assert.equal(r.status, 409);
-    assert.equal(r.body.error.code, "ENORUN");
+    assert.equal(r.status, 400);
+    assert.equal(r.body.error.code, "EUNSUPPORTED");
     await request("DELETE", `/api/lanes/${id}`);
   });
 });