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.

AGENTS.md

@@ -1,5 +1,18 @@
 # AGENTS.md
 
+## STOP — coding paused (Phase 2: requirements reconciliation)
+
+Do not implement Milestone 3 or any feature right now. Trunk docs are being consolidated.
+Scope authority is `docs/REQUIREMENTS.md` + `docs/MILESTONES.md`. M3 will be re-derived as a subset of requirement IDs.
+The Milestone 3 text below is the pre-reconciliation state and is not authoritative until Phase 2 completes.
+
+## Structure contract (read before creating any doc)
+
+Requirements live only in `docs/REQUIREMENTS.md`; milestone scope in `docs/MILESTONES.md`.
+Before creating or moving any doc, look up its home in `docs/STRUCTURE.md` (artifact registry + namespace routing).
+If no home exists, stop and ask Ty — do not create a new artifact.
+Run `python tools/check_requirements.py` after any docs change; it must pass.
+
 ## Agent context
 
 Read `docs/README.md` first, then this file, `docs/CAPTURE.md`, `docs/PHOTOS.md`, and `docs/CREATE.md`.