fix(process_registry): kill orphaned Popen on post-spawn setup failure

After Popen succeeds with os.setsid (detached process group), 5 things
happen with no try/except: Thread construction, reader.start(), lock
acquisition, prune+register, checkpoint write. If any raises, the
Popen object goes unregistered and the detached process group leaks
indefinitely.

Wrap the post-spawn setup in try/except. On failure:
  - os.killpg(getpgid(pid), SIGKILL) takes down the entire process
    group (not just the shell - important because of detached PG +
    -lic shell wrapper that may have spawned children)
  - proc.kill() fallback for ProcessLookupError/PermissionError/OSError
  - proc.wait(timeout=5) reaps with a bound
  - re-raise to preserve original traceback
Nested try/except around cleanup so a secondary failure can't mask the
original.

Closes #2749.

tests/tools/test_process_registry.py

@@ -530,6 +530,96 @@ class TestSpawnEnvSanitization:
         assert env.commands[2][0] == "cat '/path with spaces/hermes_bg.exit' 2>/dev/null"
 
 
+# =========================================================================
+# Popen leak prevention
+# =========================================================================
+
+class TestPopenLeakOnSetupFailure:
+    """Regression for issue #2749: subprocess orphaned when post-Popen setup raises."""
+
+    def test_popen_killed_when_thread_creation_fails(self, registry):
+        """If Thread() raises after Popen, proc must be killed — not orphaned."""
+        killed = []
+
+        proc = MagicMock()
+        proc.pid = 9999
+        proc.stdout = iter([])
+        proc.stdin = MagicMock()
+        proc.poll.return_value = None
+
+        def fake_kill():
+            killed.append(True)
+
+        proc.kill = fake_kill
+        proc.wait = MagicMock()
+
+        def boom(*args, **kwargs):
+            raise RuntimeError("Thread creation failed")
+
+        with patch("tools.process_registry._find_shell", return_value="/bin/bash"), \
+             patch("subprocess.Popen", return_value=proc), \
+             patch("threading.Thread", side_effect=boom), \
+             patch.object(registry, "_write_checkpoint"):
+            with pytest.raises(RuntimeError, match="Thread creation failed"):
+                registry.spawn_local("echo hello", cwd="/tmp")
+
+        assert killed, "proc.kill() must be called when post-Popen setup raises"
+
+    def test_popen_killed_when_write_checkpoint_fails(self, registry):
+        """If _write_checkpoint raises after Popen, proc must still be killed."""
+        killed = []
+
+        proc = MagicMock()
+        proc.pid = 8888
+        proc.stdout = iter([])
+        proc.stdin = MagicMock()
+        proc.poll.return_value = None
+
+        def fake_kill():
+            killed.append(True)
+
+        proc.kill = fake_kill
+        proc.wait = MagicMock()
+
+        fake_thread = MagicMock()
+
+        with patch("tools.process_registry._find_shell", return_value="/bin/bash"), \
+             patch("subprocess.Popen", return_value=proc), \
+             patch("threading.Thread", return_value=fake_thread), \
+             patch.object(registry, "_write_checkpoint", side_effect=OSError("disk full")):
+            with pytest.raises(OSError, match="disk full"):
+                registry.spawn_local("echo hello", cwd="/tmp")
+
+        assert killed, "proc.kill() must be called when _write_checkpoint raises"
+
+    def test_popen_not_killed_on_success(self, registry):
+        """Successful spawn must NOT kill the process."""
+        killed = []
+
+        proc = MagicMock()
+        proc.pid = 7777
+        proc.stdout = iter([])
+        proc.stdin = MagicMock()
+        proc.poll.return_value = None
+
+        def fake_kill():
+            killed.append(True)
+
+        proc.kill = fake_kill
+        proc.wait = MagicMock()
+
+        fake_thread = MagicMock()
+
+        with patch("tools.process_registry._find_shell", return_value="/bin/bash"), \
+             patch("subprocess.Popen", return_value=proc), \
+             patch("threading.Thread", return_value=fake_thread), \
+             patch.object(registry, "_write_checkpoint"):
+            session = registry.spawn_local("echo hello", cwd="/tmp")
+
+        assert not killed, "proc.kill() must NOT be called on successful spawn"
+        assert session.pid == 7777
+
+
 # =========================================================================
 # Checkpoint
 # =========================================================================