chore: add commit message normalizer and validator

[gregggreg]

Standardizes commit messages on Conventional Commits with a documented convention, an opt-in git hook, and CI enforcement.

Format

type(scope)!: summary

Types: build, chore, ci, docs, feat, fix, perf, refactor, revert, style, test. Scope and ! are optional. Subjects are lowercase imperative, no more than 72 chars, no trailing period, blank line before the body.

What's here

File	Purpose
docs/commit-conventions.md	The convention, in full
scripts/commit-msg-lib.sh	Shared parsing helpers (bash 3.2 compatible)
scripts/normalize-commit-msg.sh	Rewrites a message file in place
scripts/validate-commit-msg.sh	One diagnostic line per violated rule
.githooks/commit-msg	Normalize, then validate
scripts/install-hooks.sh	make install-hooks -> git config core.hooksPath .githooks
tests/commit-msg-test.sh	49 table-driven checks, no framework
.github/workflows/commit-lint.yml	Runs the suite and validates every subject the PR adds

Opt-in, and conservative by design

Hooks are opt-in via core.hooksPath - nothing changes for contributors who don't run make install-hooks. CI still checks every PR, so enforcement doesn't depend on local setup.

Normalization only rewrites what is mechanically safe: whitespace, casing of a recognized type, a trailing period, and the subject/body blank line. It never invents a type unless you explicitly pass --infer. Merge, revert, fixup!/squash!/amend!, stash subjects, and comment-only templates are passed through untouched - a hook that blocks git merge is worse than an inconsistent history.

Git's scissors marker and everything below it are copied through byte for byte, so a git commit -v diff can never leak into the commit body. An end-to-end test drives a real git commit -v through the installed hook and asserts the stored message.

Verification

• ./tests/commit-msg-test.sh - 49/49 pass on bash 3.2 (macOS system bash)
• go build ./... and go test ./... - clean
• The new scissors and end-to-end cases were confirmed to fail against the pre-fix normalizer

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

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