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>

installer/install_common.py

@@ -73,6 +73,7 @@ def write_agent_env(install_dir_path: Path, enroll_result: dict) -> None:
     lines = [
         (f"GTI_HUB_URL={enroll_result.get('hub_url') or hub_url()}"),
         (f"AGENT_ID={enroll_result.get('agent_id') or ''}"),
+        (f"INSTALLATION_ID={enroll_result.get('installation_id') or ''}"),
         ("COMPANY_ID="),
         ("COMPANY_UF="),
         ("FIREBIRD_DATABASE_PATH="),
@@ -182,4 +183,4 @@ def audit_event(agent_token: str, hub: str, event_type: str, message: str) -> No
         )
         response.raise_for_status()
     except Exception:  # noqa: BLE001 — best effort only
-        return
+        return