refactor(agent): extract agent+shared into standalone packages with full GRACE semantic markup

- Move agent code from backend/src/agent/ to agent/src/ss_tools/agent/
- Extract shared stdlib-only utilities to shared/src/ss_tools/shared/
- Add #region/#endregion contracts to all ~140 functions (INV_1 compliance)
- Update docker files, entrypoint, build scripts for new package layout
- Backend now imports ss_tools.shared._llm_health (no gradio/langchain deps)
- Add specs for 036-039 feature plans

backend/src/api/routes/__init__.py

@@ -22,6 +22,7 @@ __all__ = [
     "admin",
     "admin_api_keys",
     "agent_conversations",
+    "agent_status",
     "agent_superset",
     "agent_superset_explore",
     "assistant",

backend/src/api/routes/agent_status.py

@@ -2,7 +2,7 @@
 # #region Api.Agent.Status [C:2] [TYPE Module] [SEMANTICS api,agent,llm,status,health]
 # @BRIEF Agent LLM provider health status endpoint — used by frontend for provider availability indicator.
 # @RATIONALE Frontend performs health check at mount and auto-retries every 30s if provider unavailable.
-# @RELATION DEPENDS_ON -> [AgentChat.GradioApp]
+# @RELATION DEPENDS_ON -> [ss_tools.shared._llm_health]
 from fastapi import APIRouter
 
 router = APIRouter(prefix="/api/agent", tags=["agent-status"])
@@ -13,10 +13,14 @@ router = APIRouter(prefix="/api/agent", tags=["agent-status"])
 # @BRIEF Return cached LLM provider health status (or trigger probe if cache expired).
 # @POST Returns {"status": "ok"|"unavailable"|"timeout"|"auth_error",
 #                "last_error": str, "retry_after_s": int}
+# @RATIONALE Uses ss_tools.shared._llm_health instead of src.agent._llm_health
+#           because the agent code has been moved to a separate project. The shared
+#           module uses only openai+httpx (no gradio, no langchain), so it works in
+#           both backend and agent containers.
 @router.get("/llm-status")
 async def get_llm_status():
     """Get cached LLM provider health status. Probes provider if cache expired."""
-    from src.agent._llm_health import _check_llm_provider_health, _llm_status
+    from ss_tools.shared._llm_health import _check_llm_provider_health, _llm_status
     status = await _check_llm_provider_health()
     return {
         "status": status,