build: add commit message tooling

Add a repository-local tool to lint, format, and safely reword commit
messages. Preserve markdown lists, quotes, code blocks, and trailers
while enforcing subject and body width limits.

Expose linting, formatting, and rewording through Make targets and
document the supported workflows. Run the linter in CI for pull request
and push commit ranges so malformed messages are caught before merging.

docs/commit-tooling.md

@@ -0,0 +1,30 @@
+# Commit Message Tooling
+
+Use `scripts/commit_message.py` to lint, format, and safely reword commit
+messages. The script enforces subject/body wrapping (`69`/`72`), keeps real
+newlines, and preserves markdown-like body structure (lists, quotes, fenced
+blocks, trailers).
+
+## Common Workflows
+
+```bash
+# Lint the current commit message
+make commitmsg-lint commit=HEAD
+
+# Lint a commit range
+make commitmsg-lint range=upstream/master..HEAD
+
+# Format a message file in place
+make commitmsg-fmt file=/tmp/msg inplace=1
+
+# Reword a commit from formatted output
+make commitmsg-reword commit=<sha>
+
+# Preview a reword without rewriting history
+make commitmsg-reword commit=<sha> dryrun=1
+```
+
+## Literal Newlines
+
+If a message was created with literal `\n` sequences, use `decode=1` with
+`commitmsg-fmt` or `commitmsg-reword` to convert them to real line breaks.