chore: normalize commit messages

[gregggreg]

Summary

• normalize and validate Conventional Commit subjects in place
• preserve message bodies, trailers, and Git-generated workflow messages
• honor Git-generated comments using core.commentChar, core.commentString, or the default # prefix
• enable repository-managed pre-commit and commit-msg hooks through .githooks
• document the accepted format and add portable shell regression coverage

Format

Supported subjects use type(scope)!: summary, with optional scope and breaking-change marker. Supported types are build, chore, ci, docs, feat, fix, perf, refactor, style, and test.

The normalizer lowercases supported types, normalizes separator spacing, collapses repeated subject whitespace, trims surrounding blank lines, and removes trailing whitespace. Merge, revert, autosquash, stash, and comment-led Git-generated messages are preserved byte for byte.

Validation

• make test (all Go tests and 16 normalizer regressions)
• go vet ./...
• go build ./...
• sh -n scripts/normalize-commit-message.sh tests/commit-message-normalizer.sh .githooks/commit-msg .githooks/pre-commit
• dash -n scripts/normalize-commit-message.sh tests/commit-message-normalizer.sh .githooks/commit-msg .githooks/pre-commit
• normalizer regression suite under dash, including idempotence
• git diff --check

Stack

This targets docs-backfill because the task required branching from the original checked-out branch. It should be reviewed after #104.