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.

docs/_templates/requirement-row.md

@@ -0,0 +1,6 @@
+<!--
+Template: one requirement row.
+Append to the correct namespace section of docs/REQUIREMENTS.md.
+Do not define this requirement anywhere else.
+-->
+| `<PREFIX>-<n>` | <one testable statement> | `<shipped|M3|later>` | `<repo-relative test path>` or `—` |