feat(host-agent): add single-instance lock to prevent duplicate-process dispatch races

Acquire an exclusive, non-blocking filelock on the identity state directory
as the first action of create_application(), before resolve_host_identity()
or any enrollment/heartbeat side effect. A second process against the same
identity_path exits immediately with InstanceAlreadyRunningError naming the
lock path; the lock releases automatically on any process exit (including
SIGKILL) via OS-level advisory locking, and explicitly during run_async()'s
shutdown finally block. filelock is promoted from transitive to direct
dependency (version unchanged at 3.29.7).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/device-host-agent/tests/test_cli.py

@@ -5,6 +5,7 @@ import sys
 import pytest
 
 from host_agent import cli
+from host_agent.instance_lock import InstanceAlreadyRunningError
 from host_agent.local_account import LocalAccountStore
 
 
@@ -60,7 +61,9 @@ def test_existing_account_fast_path_skips_prompting(monkeypatch, tmp_path) -> No
     assert captured["config"].display_name == "operator"
 
 
-def test_interactive_first_run_prompts_and_creates_account(monkeypatch, tmp_path) -> None:
+def test_interactive_first_run_prompts_and_creates_account(
+    monkeypatch, tmp_path
+) -> None:
     _set_env(monkeypatch, tmp_path)
     monkeypatch.setattr(sys.stdin, "isatty", lambda: True)
     inputs = iter(["operator"])
@@ -120,3 +123,26 @@ def test_setup_subcommand_refuses_overwrite_without_confirmation(
     cli.main(["setup"])
 
     assert store.load() == original
+
+
+def test_duplicate_instance_exits_with_clear_error(
+    monkeypatch, tmp_path, capsys
+) -> None:
+    _set_env(monkeypatch, tmp_path)
+    LocalAccountStore(tmp_path / "account.json").create(
+        "operator", "correct horse battery staple"
+    )
+    lock_path = tmp_path / "identity_dir" / "host_agent.lock"
+
+    def raising_create_application(*, config=None, **kwargs):
+        raise InstanceAlreadyRunningError(lock_path)
+
+    monkeypatch.setattr(cli, "create_application", raising_create_application)
+
+    with pytest.raises(SystemExit) as exc_info:
+        cli.main([])
+
+    assert exc_info.value.code == 1
+    err = capsys.readouterr().err
+    assert "another Host Agent instance" in err
+    assert str(lock_path) in err