phase 2 (part 5): MCP server with official Go SDK

- MCP server at /mcp using github.com/modelcontextprotocol/go-sdk v1.6.1
  with Streamable HTTP transport
- 9 tools implemented: get_entity, list_entities, get_relations,
  get_blast_radius, get_health_summary, get_audit_trail,
  search_knowledge, query_metrics, request_execution (policy-gated)
- All tools use the untyped ToolHandler pattern with raw JSON
  argument parsing
- Mounted on the same binary and port with bearer/OIDC auth
- SSE stream endpoint now handled by oapi-codegen strict handler

Phase 2 acceptance criteria:
- GET /entities?type=service ✅ (part 1)
- MCP list_entities ✅ (part 5)
- PATCH /entities with If-Match → 412 ✅ (part 3)
- Idempotency-Key replay ✅ (part 3)
- SSE stream shows events ✅ (part 4)
- Audit rows carry OIDC sub ✅ (part 4)
- Spec-conformance tests ⬜ (CI setup, Phase 2 completing)

Remaining stubs: CreateEntityType, PatchEntityType, CreateCheck,
PatchCheck, RequestExecution, CancelExecution, ListApprovals,
DecideApproval, ListExecutions, GetExecution, ListPatterns,
PatchPattern, ListSkills, PatchSkill, etc. (Phase 3)

go.sum

@@ -19,6 +19,10 @@ github.com/go-openapi/testify/v2 v2.4.0 h1:8nsPrHVCWkQ4p8h1EsRVymA2XABB4OT40gcvA
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@@ -34,6 +38,8 @@ github.com/kr/pretty v0.3.0 h1:WgNl7dwNpEZ6jJ9k1snq4pZsg7DOEN8hP9Xw0Tsjwk0=
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@@ -48,16 +54,28 @@ github.com/rogpeppe/go-internal v1.15.0 h1:D0RCU5rMAp+SpgkiNdrjfJ+LX4J1M32V2NeCY
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