Fixes#915
`gt convoy check` was failing to detect closed beads in external rig databases,
causing convoys to remain perpetually open despite tracked work being completed.
Changes:
- Modified getTrackedIssues() to parse external:rig:id format and track rig ownership
- Added getExternalIssueDetails() to query external rig databases by running bd show
from the rig directory
- Changed from issueIDs []string to issueRefs []issueRef struct to track both ID and
rig name for each dependency
The fix enables proper cross-rig convoy completion by querying the appropriate database
(town or rig) for each tracked bead's status.
Testing: Verified that convoy hq-cv-u7k7w tracking external:claycantrell:cl-niwe now
correctly detects the closed status and auto-closes the convoy.
377b4877cd