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>

mcp/Dockerfile

@@ -0,0 +1,17 @@
+FROM python:3.12-slim
+
+WORKDIR /app
+
+COPY requirements.txt ./
+RUN pip install --no-cache-dir -r requirements.txt
+
+COPY *.py ./
+
+ENV KB_ENGINE_URL=http://engine:8000
+ENV KB_API_KEY=
+ENV KB_MCP_API_KEY=
+ENV KB_MCP_PORT=3000
+
+EXPOSE 3000
+
+CMD ["python", "server.py"]