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/internal/api/client.go

@@ -217,6 +217,20 @@ func (c *Client) Put(path string, body interface{}) (*http.Response, error) {
 	return c.do(req)
 }
 
+// Patch performs a PATCH request with a JSON body.
+func (c *Client) Patch(path string, body interface{}) (*http.Response, error) {
+	data, err := json.Marshal(body)
+	if err != nil {
+		return nil, fmt.Errorf("failed to marshal request body: %w", err)
+	}
+	req, err := c.newRequest(http.MethodPatch, path, bytes.NewReader(data))
+	if err != nil {
+		return nil, err
+	}
+	req.Header.Set("Content-Type", "application/json")
+	return c.do(req)
+}
+
 // DecodeJSON reads the response body and decodes it into target.
 func DecodeJSON(resp *http.Response, target interface{}) error {
 	defer resp.Body.Close()