feat(commits): add Conventional Commits message normalizer

[iamlasse]

Summary

Standardizes commit messages across the repo with a small, dependency-free normalizer plus tooling around it.

• internal/commits — pure Normalize(msg) (string, error) and Validate(msg) error enforcing Conventional Commits:
  • header type(scope)?: subject restricted to a known type set: feat, fix, docs, style, refactor, test, chore, perf, build, ci, revert
  • auto-fixes: lowercase type and subject, whitespace trimming, trailing-period removal, blank-line collapsing, 72-column body wrapping
  • rejects with actionable errors: empty message, missing/unknown type, missing subject, subject over 72 chars
  • idempotent: Normalize(Normalize(m)) == Normalize(m)
• nightshift commit normalize — cobra subcommand reading the message from a positional arg, --file, or stdin; with --file the normalized message is written back in place, otherwise printed to stdout. --check validates only, printing a diff-style report and exiting non-zero without modifying anything.
• scripts/commit-msg.sh — installable git commit-msg hook that normalizes the message file in place and rejects unfixable messages; make install-hooks now installs it alongside the pre-commit hook:

  ln -s ../../scripts/commit-msg.sh .git/hooks/commit-msg

• Docs — docs/commit-messages.md covers the format rules, examples, CLI usage, and hook installation; the README links to it.

Testing

• gofmt -l . clean, go vet ./... clean, go build ./... succeeds
• go test ./internal/commits/... ./cmd/nightshift/... passes (table-driven tests: idempotency, casing, wrapping, all error cases, Validate)
• Smoke-tested stdin/--file/--check flows and the hook itself (this commit was normalized by it)

Nightshift-Task: commit-normalize
Nightshift-Ref: https://github.com/marcus/nightshift

[iamlasse]

Review fixes pushed (35b5ed6):

Hook fallback bug (confirmed): scripts/commit-msg.sh now stores the nightshift command as a bash array so the multi-word go run ... fallback expands correctly, probes that the tool actually runs before inspecting the message, and clearly distinguishes "failed to run nightshift (tooling problem)" from "message does not follow Conventional Commits". Verified live: with no nightshift on PATH, the hook builds via go run and normalizes the message file; a broken binary reports the tooling error, not a format rejection.

Minor items:

• The fixed /tmp/nightshift-commit-msg.err is replaced by mktemp + trap cleanup.
• The 72-char subject limit is now checked after trailing-period trimming, so a 73-char subject that shrinks to 72 is accepted.
• Conventional Commits breaking-change markers (feat!:, feat(scope)!:) are accepted and preserved; documented in docs/commit-messages.md.
• Body wrap width is measured in runes, not bytes.
• Bonus fix found while committing through the hook: trailing trailer/footer blocks (Reviewed-by:, BREAKING CHANGE:, Nightshift-Task:, ...) are now preserved verbatim instead of being re-wrapped and corrupted.

gofmt/vet/build/tests all pass; new table-driven tests cover each fix.