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

client/cmd/updatenote.go

@@ -0,0 +1,61 @@
+package cmd
+
+import (
+	"fmt"
+	"os"
+	"strconv"
+
+	"github.com/kb-search/kb/internal/api"
+	"github.com/kb-search/kb/internal/output"
+	"github.com/spf13/cobra"
+)
+
+var updatenoteCmd = &cobra.Command{
+	Use:   "updatenote <id> <text>",
+	Short: "Update an existing note's content",
+	Args: func(cmd *cobra.Command, args []string) error {
+		if len(args) < 2 {
+			return fmt.Errorf("requires document ID and text arguments\n\n  Usage: kb updatenote 42 \"updated note text\"")
+		}
+		if _, err := strconv.Atoi(args[0]); err != nil {
+			return fmt.Errorf("document ID must be an integer, got %q", args[0])
+		}
+		return nil
+	},
+	RunE: runUpdatenote,
+}
+
+func init() {
+	rootCmd.AddCommand(updatenoteCmd)
+}
+
+func runUpdatenote(cmd *cobra.Command, args []string) error {
+	docID := args[0]
+	text := args[1]
+
+	client := api.NewClient()
+
+	body := map[string]string{"text": text}
+	resp, err := client.Patch(fmt.Sprintf("/api/v1/notes/%s", docID), body)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+
+	if err := api.CheckError(resp); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+
+	var result interface{}
+	if err := api.DecodeJSON(resp, &result); err != nil {
+		return fmt.Errorf("failed to decode response: %w", err)
+	}
+
+	if output.IsJSON() {
+		output.PrintJSON(result)
+	} else {
+		fmt.Printf("Updated note %s\n", docID)
+	}
+	return nil
+}