[2027-07-05] Retire master mirror + tighten workflow triggers to main-only

[sebastienrousseau]

Marker to close out the master mirror set up during the July 2026 default-branch rename (PR #961). The mirror workflow has kept raw.githubusercontent.com/…/master/… URLs alive for the 12-month grace period so bookmarked install commands don't 404.

By this date the grace window is up — external users have had a full year to re-copy the current main-based install command.

Cutover checklist (in order)

• Verify no in-flight PRs still target master (there shouldn't be any — the branch is only being written to by the mirror).
• Delete the mirror workflow — .github/workflows/mirror-main-to-master.yml.
• Delete the remote master branch:
  git push origin --delete master
• Sweep workflow triggers — remove , master from every branches: [main, master] list in .github/workflows/*.yml (should be 20 files; grep \[main, master\] to enumerate).
• Rebuild the docs site to confirm nothing regressed:
  mkdocs build --clean + pages.yml completes.
• Post-cleanup smoke:
  curl -sI https://raw.githubusercontent.com/sebastienrousseau/dotfiles/master/install.sh — should now return 404 (expected).
  curl -sI https://raw.githubusercontent.com/sebastienrousseau/dotfiles/main/install.sh — should return 200.
• Close this issue.

Context

The grace-period [main, master] triggers were added in commit 9ed7a439 (July 2026) so the migration PR itself could be tested against master before the rename. Post-rename they stayed as a defensive net; retiring them tightens CI to only the current default branch.

The mirror workflow itself was added in the same PR (#961). Its full history + rationale is in that PR's body.

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Set as a self-reminder on 2026-07-05 with a 12-month lead per the rename runbook.