fix: persist and recover the Agent's real installation_id (heartbeat activation)

POST /v1/agents/enroll now returns installation_id; new installs persist it
to agent.env immediately. Runtime recovery precedence: existing SQLite
installation_identity -> configured INSTALLATION_ID -> authenticated
GET /v1/agents/me/identity. Recovery fails closed on missing/malformed/
mismatched identity rather than inventing a UUID (which the Hub would then
reject as a conflicting Installation on an already-enrolled Agent). An
explicit AGENT_JOBS_ENABLED gate stops the service from claiming a job right
after the first heartbeat.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

agent/tests/test_windows_service_host.py

@@ -0,0 +1,58 @@
+import importlib.util
+import sys
+from pathlib import Path
+from types import SimpleNamespace
+
+
+SERVICE_WRAPPER = Path(__file__).parents[1] / "service" / "gti_agent_windows_service.py"
+
+
+def load_service_wrapper():
+    spec = importlib.util.spec_from_file_location("gti_agent_windows_service_test", SERVICE_WRAPPER)
+    module = importlib.util.module_from_spec(spec)
+    sys.modules[spec.name] = module
+    spec.loader.exec_module(module)
+    return module
+
+
+def test_main_enters_scm_dispatcher_when_started_without_command(monkeypatch):
+    service = load_service_wrapper()
+    calls = []
+
+    class FakeServiceManager:
+        def Initialize(self):
+            calls.append("initialize")
+
+        def PrepareToHostSingle(self, service_class):
+            calls.append(("prepare", service_class))
+
+        def StartServiceCtrlDispatcher(self):
+            calls.append("dispatcher")
+
+    def forbidden_handle_command_line(service_class):
+        raise AssertionError("SCM start must not be parsed as a CLI command")
+
+    monkeypatch.setattr(service, "servicemanager", FakeServiceManager())
+    monkeypatch.setattr(service, "win32serviceutil", SimpleNamespace(HandleCommandLine=forbidden_handle_command_line))
+    monkeypatch.setattr(service.sys, "argv", [str(SERVICE_WRAPPER)])
+
+    service.main()
+
+    assert calls == ["initialize", ("prepare", service.GTIFiscalAgentService), "dispatcher"]
+
+
+def test_main_preserves_pywin32_cli_commands(monkeypatch):
+    service = load_service_wrapper()
+    called = []
+
+    class ForbiddenServiceManager:
+        def __getattr__(self, name):
+            raise AssertionError("CLI commands must not enter the SCM dispatcher")
+
+    monkeypatch.setattr(service, "servicemanager", ForbiddenServiceManager())
+    monkeypatch.setattr(service, "win32serviceutil", SimpleNamespace(HandleCommandLine=lambda cls: called.append(cls)))
+    monkeypatch.setattr(service.sys, "argv", [str(SERVICE_WRAPPER), "start"])
+
+    service.main()
+
+    assert called == [service.GTIFiscalAgentService]