Add capability registry scaffold and seed entries from reuse-surface

Bootstrap registry/indexes/capabilities.yaml and migrate helix_forge
capability entries owned by this repository for federation publishing.

registry/indexes/capabilities.yaml

@@ -0,0 +1,50 @@
+version: 1
+updated: '2026-06-16'
+domain: helix_forge
+capabilities:
+- id: capability.feature-control.evaluate
+  name: Feature Availability Evaluation
+  summary: Evaluate whether a feature is active, hidden, disabled, or unavailable
+    for a subject in context.
+  vector: D5 / A4 / C3 / R3
+  domain: helix_forge
+  status: draft
+  owner: feature-control
+  path: registry/capabilities/capability.feature-control.evaluate.md
+  tags:
+  - feature-control
+  - evaluation
+  - sdk
+  consumption_modes:
+  - SDK
+  - service API
+- id: capability.feature-control.rollout
+  name: Feature Rollout Control
+  summary: Gradually expose features to subjects across tenants, domains, groups,
+    or cohorts using rollout rules.
+  vector: D4 / A2 / C2 / R1
+  domain: helix_forge
+  status: draft
+  owner: feature-control
+  path: registry/capabilities/capability.feature-control.rollout.md
+  tags:
+  - feature-control
+  - rollout
+  - planning
+  consumption_modes:
+  - source module
+  - SDK
+- id: capability.feature-control.visibility
+  name: Feature Visibility Control
+  summary: Control whether features are visible or hidden for subjects without changing
+    entitlement or authorization.
+  vector: D4 / A2 / C2 / R1
+  domain: helix_forge
+  status: draft
+  owner: feature-control
+  path: registry/capabilities/capability.feature-control.visibility.md
+  tags:
+  - feature-control
+  - visibility
+  consumption_modes:
+  - source module