CI has no path filters, so a docs commit runs the whole suite #169

.gitbay/ci.yml jobs take steps, schedule and tags. There is no way to say a job only matters for some paths, so every push to a branch queues every push job — a one-line prose fix runs the full e2e suite.

This blocks moving wikis into the repository (#170): under that design a typo fix is a commit on the default branch, and a twenty-minute build on a typo is how people learn to stop editing the wiki.

Add paths and paths-ignore globs to the job schema. gitutil.DiffFiles(dir, old, new) already produces the changed-file list.

Fail open, in every ambiguous case: a new branch with no diff base, a DiffFiles error, or a job with neither key runs the job. A filter that silently skips CI when it cannot tell is worse than no filter.

Note QueueBranchBuilds is shared with the merge path, which moves a ref without reaching a hook, so the old sha has to reach both callers.