fix(cli): clear a stale run-activity marker when spawning the binary

Review feedback on #1258: if the binary dies via SIGKILL or a native
crash, `process.on('exit', clear)` never runs and the marker outlives it
in tmpdir. Reach that pid again and the leaked file stalls the new run's
updates for the whole RUN_IDLE_MAX_WAIT_MS bound.

Clear it at spawn rather than stamping the marker with an identity to
cross-check. At the moment spawnInstalledBinary has the child's pid, the
binary has not booted, let alone started a turn -- so a marker at that
path is definitionally someone else's, and no session id is needed to
tell the two apart.

Claude-Session: https://claude.ai/code/session_018vPhyqaaoKa8cgs7GEnyq5

Author: kavish-19

cli/release-core/launcher.js

@@ -941,6 +941,22 @@ function createLauncher(productConfig) {
     return path.join(os.tmpdir(), `codebuff-run-active-${pid}`)
   }
 
+  /**
+   * Drop a marker left behind by a process that died before it could clear
+   * its own -- SIGKILL, a native crash -- since its exit handler never ran.
+   * Called with a pid we have only just spawned, so any marker at that path
+   * belongs to an earlier process the OS has since reused the pid for; the
+   * binary cannot have started a turn yet. Without this, that stale file
+   * stalls the new run's updates for the whole RUN_IDLE_MAX_WAIT_MS bound.
+   */
+  function clearStaleRunActivityMarker(pid) {
+    try {
+      fs.rmSync(runActivityMarkerPath(pid), { force: true })
+    } catch {
+      // Best effort: a marker we can't remove only costs us the bounded wait.
+    }
+  }
+
   const RUN_IDLE_POLL_INTERVAL_MS = 1_000
   // Don't stall an update behind one long-running turn forever; fall back to
   // today's immediate-restart behavior once this elapses.
@@ -1305,6 +1321,8 @@ function createLauncher(productConfig) {
     child.on('error', exitOnSpawnFailure)
     child.launch = watchLaunch(child)
 
+    if (child.pid !== undefined) clearStaleRunActivityMarker(child.pid)
+
     return child
   }
 
@@ -1500,6 +1518,7 @@ function createLauncher(productConfig) {
       ensureBinaryReady,
       isTargetAllowedForThisMachine,
       runActivityMarkerPath,
+      clearStaleRunActivityMarker,
       waitForRunIdle,
       CONFIG,
     },

cli/src/__tests__/release/wrapper-safety.test.ts

@@ -247,6 +247,57 @@ describe('shared release launcher safety', () => {
     }
   })
 
+  test('a spawned binary starts from a clean activity marker', () => {
+    const { clearStaleRunActivityMarker, runActivityMarkerPath } =
+      createLauncher({
+        packageName: 'test',
+        displayName: 'Test',
+      }).__testing
+    const pid = 999_999_004
+    const markerPath = runActivityMarkerPath(pid)
+
+    // A marker left by a process that died before it could clear its own --
+    // SIGKILL, a native crash -- outlives it in tmpdir. Reaching the same pid
+    // again would otherwise stall that run's updates for the full bound.
+    writeFileSync(markerPath, '')
+
+    try {
+      clearStaleRunActivityMarker(pid)
+      expect(existsSync(markerPath)).toBe(false)
+    } finally {
+      rmSync(markerPath, { force: true })
+    }
+  })
+
+  test('clearing a stale marker tolerates there being none', () => {
+    const { clearStaleRunActivityMarker, runActivityMarkerPath } =
+      createLauncher({
+        packageName: 'test',
+        displayName: 'Test',
+      }).__testing
+    const pid = 999_999_005
+
+    rmSync(runActivityMarkerPath(pid), { force: true })
+
+    expect(() => clearStaleRunActivityMarker(pid)).not.toThrow()
+  })
+
+  test('spawnInstalledBinary clears the stale marker once it has a pid', () => {
+    const source = readFileSync(launcherPath, 'utf8')
+    const spawnFunction = source.slice(
+      source.indexOf('function spawnInstalledBinary'),
+    )
+    const spawnIndex = spawnFunction.indexOf('child = spawn(CONFIG.binaryPath')
+    const clearIndex = spawnFunction.indexOf('clearStaleRunActivityMarker(')
+    const returnIndex = spawnFunction.indexOf('return child')
+
+    expect(spawnIndex).toBeGreaterThan(-1)
+    // The pid only exists after the spawn, and the marker must be gone before
+    // the caller can hand this child to checkForUpdates.
+    expect(clearIndex).toBeGreaterThan(spawnIndex)
+    expect(returnIndex).toBeGreaterThan(clearIndex)
+  })
+
   test('requires the wrapper release only for missing or older binaries', () => {
     const cases: Array<{
       wrapperVersion: string