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>

central/tests/test_install_code_enrollment.py

@@ -128,6 +128,7 @@ def test_enroll_with_install_code_alone_creates_agent(hub, release_id):
     assert enrolled.status_code == 200
     body = enrolled.json()
     assert body["agent_id"] and body["agent_token"] and body["hostname_mismatch"] is False
+    assert body["installation_id"] == uuid
 
     with factory() as session:
         agent = session.get(Agent, body["agent_id"])
@@ -416,4 +417,4 @@ def test_revoke_clears_code_and_is_audited(hub, tmp_path):
         enr = session.scalar(select(AgentEnrollment).where(AgentEnrollment.enrollment_uuid == installation_uuid))
         assert enr.status == "REVOKED" and enr.install_code_hash is None
     events = {e.action for e in audits(factory)}
-    assert "INSTALLATION_CODE_CREATED" in events and "INSTALLATION_CODE_REVOKED" in events and "INSTALLATION_REVOKED" in events
+    assert "INSTALLATION_CODE_CREATED" in events and "INSTALLATION_CODE_REVOKED" in events and "INSTALLATION_REVOKED" in events