Add --include-messages option to filter commits by subject regex

[autumn-n]

Adds --include-messages, a regex filter on the commit subject. Used alongside --include-paths, it lets you drop commits that shouldn't show up in a release — bot pushes, direct commits without an issue link, anything that doesn't fit your team's conventions — without rewriting history.

The regex matches the subject only (everything before the first \n) so squash dumps and trailers in the body can't accidentally match. The pattern is stored as a string and compiled once inside scanCommits, mirroring how --include-paths is stored as a string array and turned into pathspec arguments at the point of use. Both filters compose: a commit must pass both to be scanned.

Examples

# Only commits that reference a Linear issue identifier
linear-release sync --include-messages="[A-Z]{2,}-[0-9]+"

# Conventional Commits — drop chore/docs/test/ci/build noise
linear-release sync --include-messages="^(feat|fix|perf):"