feat(phase-3): workstream 2 — W3C DIDs

Implements W3C DID Core 1.0 per-agent identity for every registered agent:

Schema:
- agent_did_keys table: stores EC P-256 public key JWK + Vault path for private key
- agents.did + agents.did_created_at columns

Key management:
- EC P-256 key pair generated on every agent registration via Node.js crypto
- Private key stored in Vault KV v2 (dev:no-vault marker when Vault not configured)
- Public key JWK stored in PostgreSQL agent_did_keys table

API (4 new endpoints):
- GET /.well-known/did.json — instance DID Document (public, cached)
- GET /api/v1/agents/:id/did — per-agent DID Document (public, 410 for decommissioned)
- GET /api/v1/agents/:id/did/resolve — W3C DID Resolution result (agents:read scope)
- GET /api/v1/agents/:id/did/card — AGNTCY agent card (public)

Implementation:
- DIDService: DID construction, key generation, Redis caching (TTL configurable)
- DIDController: 410 Gone for decommissioned agents, correct Content-Type on resolve
- AgentService: calls DIDService.generateDIDForAgent on every new registration

Tests: 429 passing, DIDService 98.93% coverage, private key absence verified in all responses

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

policies/data/scopes.json

@@ -18,6 +18,7 @@
     "GET:/api/v1/organizations/:id":                       ["admin:orgs"],
     "PATCH:/api/v1/organizations/:id":                     ["admin:orgs"],
     "DELETE:/api/v1/organizations/:id":                    ["admin:orgs"],
-    "POST:/api/v1/organizations/:id/members":              ["admin:orgs"]
+    "POST:/api/v1/organizations/:id/members":              ["admin:orgs"],
+    "GET:/api/v1/agents/:agentId/did/resolve":             ["agents:read"]
   }
 }