Add MCP server, note mutation endpoint, and updated_at tracking (v3.0.0)

New MCP server (mcp/) exposes kb operations as native MCP tools over
Streamable HTTP with Bearer token auth. Supports collections via tag
conventions, chunked file uploads, and agent-side search patterns.

Engine gains PATCH /api/v1/notes/{id} for in-place note updates with
transactional re-chunk/re-embed, and updated_at column on documents.

Go client adds updatenote command and Patch HTTP method.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

engine/compose.nvidia.yaml

@@ -23,3 +23,17 @@ services:
       - KB_SEARCH_THRESHOLD=${KB_SEARCH_THRESHOLD:-0.01}
       - HF_HUB_OFFLINE=${HF_HUB_OFFLINE:-}
     restart: unless-stopped
+
+  kb-mcp:
+    build:
+      context: ../mcp
+      dockerfile: Dockerfile
+    ports:
+      - "${KB_MCP_PORT:-3000}:3000"
+    environment:
+      - KB_ENGINE_URL=http://kb-engine:8000
+      - KB_API_KEY=${KB_API_KEY:-}
+      - KB_MCP_API_KEY=${KB_MCP_API_KEY:-}
+    depends_on:
+      - kb-engine
+    restart: unless-stopped