Add capability registry with seed entry from reuse-surface

Bootstrap registry layout and migrate helix_forge capability owned by
this repository (REUSE-WP-0014-T02).

registry/README.md

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+# Capability Registry
+
+Markdown-first capability index for federation and reuse planning.
+
+## Authoring
+
+1. Copy a capability entry template (see reuse-surface `templates/capability-entry.template.md`).
+2. Add the row to `indexes/capabilities.yaml`.
+3. Run `reuse-surface validate` from a checkout with the CLI installed.
+4. Merge to `main` and verify publish with `reuse-surface establish --publish-check`.
+
+Federation contract: reuse-surface `docs/RegistryFederation.md`.

registry/capabilities/capability.activity.event-coordinate.md

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+---
+id: capability.activity.event-coordinate
+name: Organizational Event Coordination
+summary: Coordinate structured responses to cross-domain events through activity workflows and automation.
+owner: activity-core
+status: draft
+domain: helix_forge
+tags: [activity, coordination, automation]
+
+maturity:
+  discovery:
+    current: D3
+    target: D5
+    confidence: medium
+    rationale: activity-core INTENT defines org-wide event response boundary.
+  availability:
+    current: A1
+    target: A4
+    confidence: low
+    rationale: Conceptual workflows exist; consumable API surface still emerging.
+
+external_evidence:
+  completeness:
+    level: C1
+    name: Fragmentary
+    confidence: low
+    basis: scope_vs_intent_and_consumer_expectations
+    satisfied_expectations:
+      - problem and boundary documented in INTENT
+    broken_expectations:
+      - no registry-native automation artifacts indexed yet
+    out_of_scope_expectations:
+      - owning domain-specific business logic
+  reliability:
+    level: R0
+    confidence: low
+    basis: consumer_quality_signals
+    known_reliability_risks: []
+
+discovery:
+  intent: >
+    Give the organization a structural home for responding to events across repos
+    and domains in an auditable, automation-ready way.
+  includes:
+    - event-triggered coordination
+    - cross-domain maintenance workflows
+  excludes:
+    - single-repo cron replacements only
+  use_cases: []
+
+availability:
+  current_level: A1
+  target_level: A4
+  current_artifacts:
+    - activity-core/INTENT.md
+  consumption_modes:
+    - informational
+
+relations:
+  depends_on: []
+  related_to:
+    - capability.statehub.workstream-coordinate
+    - capability.audit.event-retain
+
+consumer_guidance:
+  recommended_for:
+    - planning org-wide event response patterns
+  not_recommended_for:
+    - assuming production automation is available
+  known_limitations:
+    - early discovery stage
+---
+
+# Organizational Event Coordination
+
+activity-core coordinates how the org responds to events—not the domain logic
+inside each repo.

registry/indexes/capabilities.yaml

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+version: 1
+updated: '2026-06-16'
+domain: helix_forge
+capabilities:
+- id: capability.activity.event-coordinate
+  name: Organizational Event Coordination
+  summary: Coordinate structured responses to cross-domain events through activity
+    workflows and automation.
+  vector: D3 / A1 / C1 / R0
+  domain: helix_forge
+  status: draft
+  owner: activity-core
+  path: registry/capabilities/capability.activity.event-coordinate.md
+  tags:
+  - activity
+  - coordination
+  - automation
+  consumption_modes:
+  - informational