Document commit-level filters and path denylists
Filtering now has two axes: which commits are reviewed (--author,
--committer, --start-date, --end-date, --text) and which files within
them (the ignore layers, the --file/--dir allowlist, and the new
--exclude-file/--exclude-dir denylist).
Restructure Filtering.md around that split and add the details a user
can only get wrong once: --end-date includes the day named, branch-name
matching is best-effort because a squash merge destroys the association,
and the reviewed diff is a concatenation of per-commit patches so one
file can appear several times.
Also record where the filters do NOT apply and why — `commit` describes
the staged index, `remote pr` reads its diff from `gh` — and note that
`remote pr` now honours --file/--dir on the posting path, not only under
--no-post.
Global-flags, Review, Diff, Summary, Dry-run, Guard, MCP-server,
Commit, Remote-pr, JSON-schema updated to match.
Shrink `COMMITBRIEF.md` via `commitbrief compress` — [Compress command]
Add new documentation files