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/contract.md

@@ -0,0 +1,11 @@
+<!--
+Template: contract doc. Home: docs/contracts/<NAME>.md
+Cross-cutting interface detail. Cite requirement IDs; never define them.
+-->
+# <Contract>
+
+Cites: `<IDs from docs/REQUIREMENTS.md>`
+
+## <Interface detail>
+
+<schemas, endpoints, prompts, screen rules — non-normative detail that supports the cited requirements>