fix(actions): keep action run title clickable when commit subject is a URL (#37867)

- When a commit subject is a bare URL, `linkProcessor` wrapped it in its
own `<a>` to that URL. Because HTML cannot nest anchors, the wrapping
default link (the action run / commit link) was lost and the action
title became unclickable — clicking it sent the user to the URL from the
commit message instead of the action log.
- Drop `linkProcessor` from `PostProcessCommitMessageSubject` so the
whole subject stays wrapped in the default link. URLs in subjects now
render as text inside that link; URLs in commit bodies are unaffected.

Fixes #37865

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Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>