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

shared/pyproject.toml

@@ -0,0 +1,17 @@
+[build-system]
+requires = ["setuptools>=69", "wheel"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "ss-tools-shared"
+version = "0.1.0"
+description = "Shared lightweight utilities for superset-tools backend and agent"
+requires-python = ">=3.11"
+dependencies = []
+
+[project.optional-dependencies]
+llm = ["openai>=1.0.0", "httpx>=0.28.1"]
+
+[tool.setuptools.packages.find]
+where = ["src"]
+include = ["ss_tools.shared*"]