feat(gbrain-sync): self-heal stale autopilot lock (dead-pid)

detectAutopilot treated a lock FILE as proof of life, so a crashed gbrain daemon
left a stale lock that wedged every sync forever (observed: a dead pid refused
--full indefinitely). Now read the holder pid (bare or JSON body) and check
liveness via signal-0: ESRCH=dead → ignore the stale signal and keep checking;
EPERM=alive (other user) → active. A stale lock never masks a live autopilot
process. Pure decision function — does not delete the file; the caller may clean it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/gbrain-guards.test.ts

@@ -38,6 +38,45 @@ describe("detectAutopilot", () => {
     expect(r.active).toBe(false);
     expect(r.signal).toBeNull();
   });
+
+  // Stale-lock self-heal: a crashed daemon's lock (dead holder pid) must NOT
+  // wedge syncs forever (observed: dead pid refused --full indefinitely).
+  const DEAD_PID = 2999999; // above macOS pid_max; vanishingly unlikely elsewhere
+
+  test("ignores a STALE lock whose holder pid is dead", () => {
+    const tmp = fs.mkdtempSync(join(os.tmpdir(), "ap-"));
+    const lock = join(tmp, "autopilot.lock");
+    fs.writeFileSync(lock, `${DEAD_PID}\n`);
+    const r = detectAutopilot(process.env, { lockPaths: [lock], processRunning: () => false });
+    expect(r.active).toBe(false);
+    expect(r.signal).toBeNull();
+  });
+
+  test("treats a FRESH lock (live holder pid) as active", () => {
+    const tmp = fs.mkdtempSync(join(os.tmpdir(), "ap-"));
+    const lock = join(tmp, "autopilot.lock");
+    fs.writeFileSync(lock, String(process.pid)); // the test runner itself is alive
+    const r = detectAutopilot(process.env, { lockPaths: [lock], processRunning: () => false });
+    expect(r.active).toBe(true);
+    expect(r.signal).toContain(`pid ${process.pid}`);
+  });
+
+  test("parses a JSON lock body and ignores it when the pid is dead", () => {
+    const tmp = fs.mkdtempSync(join(os.tmpdir(), "ap-"));
+    const lock = join(tmp, "autopilot.lock");
+    fs.writeFileSync(lock, JSON.stringify({ pid: DEAD_PID, started_at: "x" }));
+    const r = detectAutopilot(process.env, { lockPaths: [lock], processRunning: () => false });
+    expect(r.active).toBe(false);
+  });
+
+  test("a stale lock does not mask a live autopilot process", () => {
+    const tmp = fs.mkdtempSync(join(os.tmpdir(), "ap-"));
+    const lock = join(tmp, "autopilot.lock");
+    fs.writeFileSync(lock, `${DEAD_PID}`);
+    const r = detectAutopilot(process.env, { lockPaths: [lock], processRunning: () => true });
+    expect(r.active).toBe(true);
+    expect(r.signal).toBe("process:gbrain autopilot");
+  });
 });
 
 // ── #1734 remove safety (E7: fail closed on user-managed without keep-storage) ─