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>

client/cmd/examples.go

@@ -23,6 +23,9 @@ Search:
   kb search "how to restart nginx"
   kb search "deploy" --tags ops --top 5
 
+Update notes:
+  kb updatenote 42 "revised note content"
+
 Manage documents:
   kb list --type pdf
   kb info 3