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/atom.py

@@ -178,6 +178,11 @@ def generate_atom_streaming(
         # Link to entry
         yield f'    <link rel="alternate" type="text/html" href="{_escape_xml(permalink)}"/>\n'
 
+        # Add category elements for tags (v1.3.1)
+        if hasattr(note, 'tags') and note.tags:
+            for tag in note.tags:
+                yield f'    <category term="{_escape_xml(tag["name"])}" label="{_escape_xml(tag["display_name"])}"/>\n'
+
         # Media enclosures (v1.2.0 Phase 3, per Q24 and ADR-057)
         if hasattr(note, 'media') and note.media:
             for item in note.media: