chore: pin GitHub Actions to commit SHA, remove pr-review workflow

- Pin all action references to full commit SHA instead of mutable
  version tags. Tag retained as inline comment for readability.
- Remove pr-review.yml workflow.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Guillaume Lours <glours@users.noreply.github.com>

# Conflicts:
#	.github/workflows/pr-review.yml