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/capabilities/README.md

@@ -0,0 +1,13 @@
+# Capability docs
+
+One file per capability. Behavior and acceptance only.
+
+Rules (`docs/STRUCTURE.md`): a capability doc **cites** requirement IDs from `docs/REQUIREMENTS.md`; it never defines or restates them.
+
+Expected files (Phase 2 migration):
+
+- `CAPTURE.md` — Milestone 1 Capture
+- `PHOTOS.md` — Milestone 2 Photos
+- `CREATE.md` — Milestone 3 Create
+
+Template: `docs/_templates/capability.md`.