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

docker/agent.entrypoint.sh

@@ -15,5 +15,5 @@ else
   echo "[agent-entry] certs.sh not found — skipping corporate CA installation"
 fi
 
-exec python -m src.agent.run
+exec python -m ss_tools.agent.run
 # #endregion docker.agent.entrypoint