chore: bump package.json to v6.2.0 and fix tags_file deprecation

- package.json: 6.1.0 → 6.2.0 (sync with release)
- mkdocs.yml: remove deprecated tags_file option (Material 9.6+)
  docs/tags.md already has inline <!-- material/tags --> marker

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: Test User

package.json

@@ -1,6 +1,6 @@
 {
   "name": "flow-cli",
-  "version": "6.1.0",
+  "version": "6.2.0",
   "description": "ADHD-optimized ZSH workflow plugin",
   "private": true,
   "scripts": {