feat(commits): add Conventional Commits message normalizer

[iamlasse]

Summary

Standardize the project's commit message format by enforcing Conventional Commits going forward via tooling (no history rewrite).

• internal/commits — pure Normalize/Validate functions enforcing the project's rules: known type set (feat, fix, docs, style, refactor, perf, test, chore, build, ci), lowercase type and subject, trimmed whitespace, 72-char subject limit, blank line between subject and body, body wrapped at 72 columns. Table-driven unit tests cover valid messages, type canonicalization, unknown types, over-length subjects, whitespace trimming, multi-paragraph wrapping, and empty/subject-only messages.
• nightshift commit normalize — cobra subcommand reading the message from a positional argument, --file (e.g. .git/COMMIT_EDITMSG), or stdin. --check validates without rewriting. With --file the normalized message is written back in place; non-zero exit on validation failure so hooks can block bad commits.
• scripts/commit-msg.sh — installable git commit-msg hook (falls back to go run when the binary is not on PATH); make install-hooks now installs it alongside pre-commit.
• docs/commit-messages.md — documents the enforced format, examples, CLI usage, and hook installation.

Verified end-to-end: a non-conforming message is blocked by the hook, a fixable one (uppercase type, stray whitespace/punctuation, unwrapped body) is rewritten in place before the commit is created.

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

[iamlasse]

Fixes both review blockers from iteration 1:

1. Trailer handling (wrapBody) — the final paragraph is now detected as a git trailer block (Token: value lines plus indented continuations) and emitted verbatim: trailers are never joined onto one line and never wrapped, even past 72 columns. Structured lines inside paragraphs (list items - * + 1., block quotes, indented or fenced code) also keep their own lines; only runs of prose are joined and wrapped. Documented in docs/commit-messages.md. Six new table-driven cases cover trailers, overlong trailers, continuations, lists, indented code, and prose/list interleaving.

2. Verification methodology — this fix commit (4d407e0) was created through the hook itself, with a fresh build of the binary first on PATH: the message file was drafted with an unwrapped single-line body, commit-msg reported "normalized", and git log -1 --format=%B on the resulting commit shows the body wrapped at 72 columns with both Nightshift-* trailers intact on separate lines — direct evidence the hook rewrote it. (Iteration 1's commit 1723edc was never rewritten because the stale nightshift v0.3.4 on PATH predates the subcommand; re-normalizing 1723edc's message now yields wrapped body + separate trailers.)

go build/go vet/gofmt -l clean, full go test ./... passes.