Address two Bugbot review comments from PR #191:
1. Closed MRs are now correctly reported as non-mergeable in
getMergeability, returning a "MR is closed" blocker instead of
falling through to produce an empty blockers list.
2. Extract shared glab, parseJSON, extractHost, and stripHost helpers
into scm-gitlab/src/glab-utils.ts. The tracker-gitlab plugin now
imports these from @composio/ao-plugin-scm-gitlab/glab-utils instead
of maintaining duplicate copies.
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>