chore: remove archived and 404 repos

Removes 275 entries across 48 sections where the referenced GitHub repo
is archived (234) or returns 404 / 451 (34). All removals were verified
programmatically via the GitHub REST API.

No entries were added, reordered, or reworded. Diff is 0 insertions,
276 deletions (1 extra for a continuation line on a multi-line entry).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>