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/app/agent_distribution.py

@@ -194,6 +194,7 @@ def router(session_factory):
                              target_type="agent", target_id=str(agent.id), context={"ip": ip, "installation": enr.enrollment_uuid})
                 session.commit()
                 return {"agent_id": agent.id, "agent_token": plain_token, "tenant_id": enr.tenant_id,
+                        "installation_id": enr.enrollment_uuid,
                         "hub_url": config.settings.base_url, "hostname_mismatch": mismatch}
             except enrollment.EnrollmentRejected as rejected:
                 session.rollback()