feat(commits): Conventional Commits normalizer + commit-msg hook

[iamlasse]

Summary

Adds a reusable, dependency-free Conventional Commits message normalizer, exposed as nightshift commit normalize and backed by an installable commit-msg git hook.

This is iteration 2 of the commit-normalize task. It fixes the two defects found in review of the prior attempt:

1. --check is no longer a no-op. --check now validates only — it prints an ok confirmation instead of the rewritten message, and exits non-zero on validation failure. The default (no flag) path still prints the normalized message.
2. Conventional Commits 1.0.0 breaking-change markers are accepted. feat!: and feat(api)!: now parse, validate, and re-emit correctly instead of failing with "commit type is not in the allowed set".

Changes

• internal/commits/normalizer.go — pure Normalize with table-driven validation (type set, scope, ! marker, subject length/capitalization, comment stripping, trailing whitespace/period, 72-col body wrapping).
• internal/commits/normalizer_test.go — table-driven tests + idempotency, including the breaking-change marker cases.
• cmd/nightshift/commands/commit.go — nightshift commit normalize [--check] [--file FILE] [MSG].
• scripts/commit-msg.sh — installable hook (rewrites the message file; rejects unfixable messages; uses mktemp for its scratch file).
• docs/commit-messages.md and Makefile (install-hooks) — docs + install wiring.

Verification

• `go build ./...`, `go vet ./...`, `go test ./...` all pass; `gofmt` clean.
• Behavioral checks:
  • `feat!: redesign api` and `feat(api)!: redesign api` → normalized and preserved.
  • `--check` prints `ok` (exit 0); default prints the normalized message.
  • Invalid messages exit non-zero in both modes.

Closes the commit-normalize task.