feat: Add tag/category support to all feed formats (v1.3.1)

Implements tag/category rendering in RSS 2.0, Atom 1.0, and JSON Feed 1.1
syndication feeds. Tags added in v1.3.0 are now visible to feed readers.

Changes:
- Load tags in feed generation (_get_cached_notes)
- Add <category> elements to RSS 2.0 feeds
- Add <category term/label> to Atom 1.0 feeds
- Add "tags" array to JSON Feed 1.1 items
- Omit empty tags field in JSON Feed (minimal approach)

Standards compliance:
- RSS 2.0: category element with display name
- Atom 1.0: RFC 4287 category with term and label attributes
- JSON Feed 1.1: tags array of strings

Per design: docs/design/v1.3.1/feed-tags-design.md
Implementation: docs/design/v1.3.1/2025-12-10-feed-tags-implementation.md

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

starpunk/feeds/json_feed.py

@@ -307,6 +307,12 @@ def _build_item_object(site_url: str, note: Note) -> Dict[str, Any]:
 
         item["attachments"] = attachments
 
+    # Add tags array (v1.3.1)
+    # Per spec: array of plain strings (tags, not categories)
+    # Omit field when no tags (user decision: no empty array)
+    if hasattr(note, 'tags') and note.tags:
+        item["tags"] = [tag['display_name'] for tag in note.tags]
+
     # Add custom StarPunk extensions
     item["_starpunk"] = {
         "permalink_path": note.permalink,