docs(structure): add requirements contract, job namespaces, and enforcement gate

Phase 1 scaffold for the docs consolidation:

- docs/STRUCTURE.md: artifact registry, 8 owner-job ID namespaces, routing and change rules
- docs/REQUIREMENTS.md, docs/MILESTONES.md: normative schema, empty pending Phase 2 port
- docs/_templates/: requirement-row, capability, contract, adr
- docs/capabilities/, docs/contracts/: registered homes
- tools/check_requirements.py: enforces registry, IDs, and traceability
- AGENTS.md: coding paused banner + structure-contract routing rule
- README.md: pointers to the structure/requirements/milestones set

Additive only: no doc moves, no requirement text, no CI wiring yet.

tools/README.md

@@ -0,0 +1,9 @@
+# tools
+
+Structure and verification gates. These are the only sanctioned way to claim the docs are clean or a slice is "done".
+
+| Script | Purpose | Run |
+|---|---|---|
+| `check_requirements.py` | Enforce `docs/STRUCTURE.md`: registered docs, requirement IDs, traceability. | `python tools/check_requirements.py` |
+
+Run from the repo root. Exit 0 = pass, exit 1 = fail.