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/adr.md

@@ -0,0 +1,20 @@
+<!--
+Template: decision record. One file per decision.
+Record why a choice was made when the reason is not obvious from the code or requirements.
+-->
+# ADR-<n>: <title>
+
+- Date: <yyyy-mm-dd>
+- Status: `<proposed | accepted | superseded>`
+
+## Context
+
+<the situation and forces>
+
+## Decision
+
+<what was decided>
+
+## Consequences
+
+<what this makes easier, harder, or impossible>