fix: implement tag index pages

[tylerdave]

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[github-actions]

Docs Site preview: https://50752070.rockgarden.pages.dev

[claude]

All four previously-flagged bugs are resolved in this PR:

1. Path traversal — fixed by moving normalize_tag into urls.py with a regex that strips everything outside [a-z0-9_-], including / and ..
2. clean_urls=False silently ignored for tag pages — fixed; build_tag_pages now writes tags/<slug>.html when clean_urls=False, and get_tag_url generates matching URLs.
3. & 4. Tag slug mismatch in templates — fixed by exposing normalize_tag as a Jinja2 global in engine.py and using {% set tag_slug = normalize_tag(tag) %} in both page.html and folder_index.html.

LGTM — no new significant issues found.