Fix code-review findings (changelog rendering, release skill, sidebar)

- changelog: fix 3 silent RST mis-renders carried over from the old monolith
  — Markdown link in 0.7.0, mismatched/single backticks in 0.18.0 / 0.20.0
- make-release skill contributors one-liner: parallelize the gh fetch, anchor
  the bot filter (was a substring that dropped handles like "abbott"), and join
  with ", " (paste -sd cycles the delimiter -> "@a,@b @c"); finalize now stages
  only the reviewed set (git add -A -- ':!.idea'); drop the obsolete CRLF gotcha
- make_release.py: emit LF not CRLF in the repository.yml insertion (fixes the
  gotcha at the source)
- docs sidebar: drop the hardcoded furo component list; override brand.html to
  include furo's own template + the sponsor button (decoupled from furo
  internals, no upper-bound pin needed); move sponsor styles into custom.css
- .pre-commit-config.yaml: drop stale end-of-file-fixer excludes for the
  deleted contributors/CoC include shims

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

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