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/capability.md

@@ -0,0 +1,17 @@
+<!--
+Template: capability doc. Home: docs/capabilities/<NAME>.md
+Behavior and acceptance only. Cite requirement IDs; never define them.
+-->
+# <Capability>
+
+Status: `<shipped | in progress | planned>`
+
+Cites: `<IDs from docs/REQUIREMENTS.md>`
+
+## Behavior
+
+<what the capability does, in plain terms>
+
+## Acceptance
+
+<what must be observed for this capability to be considered done, and where the evidence lives>