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/capabilities/capability.feature-control.evaluate.md

@@ -0,0 +1,138 @@
+---
+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.
+owner: feature-control
+status: draft
+domain: helix_forge
+tags:
+  - feature-control
+  - evaluation
+  - sdk
+
+maturity:
+  discovery:
+    current: D5
+    target: D7
+    confidence: medium
+    rationale: >
+      Concrete use cases and research references are documented in the maturity
+      standard example and feature-control domain work.
+  availability:
+    current: A4
+    target: A6
+    confidence: medium
+    rationale: >
+      SDK and service API artifacts exist in the feature-control repository.
+      Managed platform operation is the natural target for multi-tenant reuse.
+
+external_evidence:
+  completeness:
+    level: C3
+    name: Functional Core
+    confidence: medium
+    basis: scope_vs_intent_and_consumer_expectations
+    satisfied_expectations:
+      - tenant-level feature evaluation
+      - user-level feature evaluation
+      - machine-readable decision result
+    broken_expectations:
+      - no agent-specific rule simulation
+      - no bulk import/export of rules
+    out_of_scope_expectations:
+      - billing entitlement ownership
+      - authorization policy enforcement
+  reliability:
+    level: R3
+    name: Usable
+    confidence: medium
+    basis: consumer_quality_signals
+    known_reliability_risks:
+      - unclear timeout behavior under heavy load
+      - limited diagnostics for complex tenant rule conflicts
+
+discovery:
+  intent: >
+    Support controlled feature availability across installations, tenants,
+    domains, groups, users, and agents.
+  includes:
+    - feature decision evaluation
+    - context-aware targeting
+    - explainable decision result
+  excludes:
+    - user authorization
+    - billing entitlement ownership
+    - UI rendering
+  assumptions:
+    - subject and tenant context can be resolved by adjacent capabilities
+  use_cases:
+    - ucc.feature-control.tenant-toggle
+    - ucc.feature-control.agent-disable
+    - ucc.feature-control.domain-rollout
+  research_memos:
+    - specs/CapabilityMaturityStandard.md
+
+availability:
+  current_level: A4
+  target_level: A6
+  current_artifacts:
+    - feature-control/packages/feature-control-sdk
+    - feature-control/services/feature-control-api
+  target_artifacts:
+    - feature-control/charts/feature-control
+    - feature-control/platform/feature-control-service
+  consumption_modes:
+    - SDK
+    - service API
+    - managed platform service
+
+relations:
+  depends_on:
+    - capability.identity.vocabulary-canonicalize
+  supports:
+    - capability.registry.register
+  related_to:
+    - capability.feature-control.rollout
+    - capability.feature-control.visibility
+
+evidence:
+  documentation:
+    - specs/CapabilityMaturityStandard.md
+  tests: []
+  consumer_feedback: []
+  bug_reports: []
+  incidents: []
+
+consumer_guidance:
+  recommended_for:
+    - application integration through SDK or service API
+    - tenant-aware feature gating in helix_forge products
+  not_recommended_for:
+    - authorization decisions
+    - billing entitlement ownership
+    - assuming agent-specific simulation without checking current scope
+  known_limitations:
+    - bulk rule management is not yet covered
+    - agent-specific simulation remains a known gap
+---
+
+# Feature Availability Evaluation
+
+## Overview
+
+This capability evaluates whether a feature should be available for a subject in
+a given context. It is the primary implementation-ready example in the sample
+registry and demonstrates a D5/A4/C3/R3 vector.
+
+## Current reuse mode
+
+Consumers can integrate through SDK or service API artifacts in the
+feature-control repository. This is implementation reuse (A4), not just planning
+metadata.
+
+## Comparison notes
+
+Compared with `capability.registry.register`, this entry is far stronger on
+availability and external evidence. Compared with
+`capability.identity.vocabulary-canonicalize`, it is stronger on delivery mode
+and weaker on cross-domain generalization.