feat(tidy-commits): add commit-history tidying skill

[akunzai]

Summary

Adds the tidy-commits skill: a guide for tidying a local branch's commit history into a clear, reviewable story before review or merge — squash fixups, reword, reorder, split, or drop commits while preserving the intended final tree.

Highlights:

• Preflight + backup: determine base, fetch, require a clean state, record current state, create a backup ref before any rewrite.
• Cleanup plan: a classification table mapping each commit type to a default action.
• Non-interactive rewrite: reset --soft <base> shortcut for collapse-all, plus the GIT_SEQUENCE_EDITOR todo flow (generated oldest-first via git log --reverse) for selective fixup/reorder/split, with recovery steps.
• Stacked branches: --update-refs by default; flag refs checked out in other worktrees.
• Verification: prove the final tree matches the backup; signature handling for protected branches.
• Push safety: never plain --force; --force-with-lease only, with confirmation.

Includes a content test (tests/tidy-commits-content.sh) and a CI workflow (.github/workflows/test-tidy-commits.yml), plus a README entry.

Test plan

• bash tests/tidy-commits-content.sh passes (12 content assertions)
• CI workflow runs the same check on push/PR to main