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proposed | accepted | superseded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>

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>

docs/_templates/contract.md

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+<!--
+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>

docs/_templates/requirement-row.md

@@ -0,0 +1,6 @@
+<!--
+Template: one requirement row.
+Append to the correct namespace section of docs/REQUIREMENTS.md.
+Do not define this requirement anywhere else.
+-->
+| `<PREFIX>-<n>` | <one testable statement> | `<shipped|M3|later>` | `<repo-relative test path>` or `—` |