feat(mcp): add describe_tools and annotate_tool functionality

* Implement DescribeTool for listing available MCP tools with annotations.
* Add UpsertToolAnnotation and GetToolAnnotations methods for managing tool notes.
* Create tool_annotations table for storing tool usage notes.

internal/store/tool_annotations.go

@@ -0,0 +1,38 @@
+package store
+
+import (
+	"context"
+	"fmt"
+)
+
+func (db *DB) UpsertToolAnnotation(ctx context.Context, toolName, notes string) error {
+	_, err := db.pool.Exec(ctx, `
+		insert into tool_annotations (tool_name, notes)
+		values ($1, $2)
+		on conflict (tool_name) do update
+		  set notes      = excluded.notes,
+		      updated_at = now()
+	`, toolName, notes)
+	if err != nil {
+		return fmt.Errorf("upsert tool annotation: %w", err)
+	}
+	return nil
+}
+
+func (db *DB) GetToolAnnotations(ctx context.Context) (map[string]string, error) {
+	rows, err := db.pool.Query(ctx, `select tool_name, notes from tool_annotations`)
+	if err != nil {
+		return nil, fmt.Errorf("get tool annotations: %w", err)
+	}
+	defer rows.Close()
+
+	annotations := make(map[string]string)
+	for rows.Next() {
+		var toolName, notes string
+		if err := rows.Scan(&toolName, &notes); err != nil {
+			return nil, fmt.Errorf("scan tool annotation: %w", err)
+		}
+		annotations[toolName] = notes
+	}
+	return annotations, rows.Err()
+}