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/config.py

@@ -0,0 +1,9 @@
+"""Configuration from environment variables."""
+
+import os
+
+
+KB_ENGINE_URL = os.environ.get("KB_ENGINE_URL", "http://localhost:8000")
+KB_API_KEY = os.environ.get("KB_API_KEY", "")
+KB_MCP_API_KEY = os.environ.get("KB_MCP_API_KEY", "")
+KB_MCP_PORT = int(os.environ.get("KB_MCP_PORT", "3000"))