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/MILESTONES.md

@@ -0,0 +1,21 @@
+# Milestones
+
+A milestone is an explicit subset of requirement IDs from `docs/REQUIREMENTS.md`. No prose scope here.
+
+Populated in Phase 2. Until then every milestone is empty and coding is paused.
+
+## M1 — Capture (shipped)
+
+- IDs: —
+
+## M2 — Photos (shipped)
+
+- IDs: —
+
+## M3 — Create (paused)
+
+- IDs: —
+
+## Later
+
+- IDs: —