feat(commits): add Conventional Commits message normalizer

[iamlasse]

Summary

Standardizes commit message format with a Conventional Commits normalizer, restored from the unmerged commit c26c5fd and reviewed against main with three fixes applied.

Format rules (internal/commits)

• Header: <type>(<scope>): <subject> per Conventional Commits 1.0.0
• Allowed types: feat, fix, docs, style, refactor, test, chore, perf, build, ci
• Type auto-lowercased; capitalized subject is a hard error
• Subject max 72 chars, no trailing period, whitespace trimmed
• Body wrapped at 72 columns, paragraphs preserved, git # comments stripped
• Normalization is idempotent; sentinel errors (ErrEmptyMessage, ErrMissingType, ErrUnknownType, ErrMissingSubject, ErrSubjectTooLong, ErrSubjectLowercase) matched via errors.Is

CLI

nightshift commit normalize "feat: add login screen"   # positional
nightshift commit normalize --file .git/COMMIT_EDITMSG  # hook usage
git log -1 --pretty=%B | nightshift commit normalize     # stdin
nightshift commit normalize --check "feat: add login"   # validate only, non-zero exit on invalid

Hook

make install-hooks now installs both pre-commit and commit-msg. The commit-msg hook normalizes the message file in place before the commit is created and blocks messages it cannot fix (missing/unknown type, capitalized or overlong subject). Bypass with git commit --no-verify.

Docs: docs/commit-messages.md.

Fixes applied on top of c26c5fd

• scripts/commit-msg.sh: set -e aborted the script before the friendly error could print — capture inside if ! and use a mktemp error file instead of a fixed /tmp path
• commit normalize --check: no longer prints the rewritten message (validate-only)
• CLI errors print once without the usage block spam
• Makefile install-hooks actually links the commit-msg hook (docs referenced it but it wasn't installed)

Testing

• go build ./... ✅
• go vet ./... ✅
• gofmt -l . clean ✅
• go test ./... all packages pass ✅ (table-driven tests in internal/commits/normalizer_test.go: happy path, auto-fix cases, every sentinel error, idempotency, allowed-type set)
• Manually verified: CLI positional/--file/stdin/--check paths, and the hook rewriting a message file in place + rejecting an invalid one

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