trail show: restore per-change merge gates for project trails

Project-trail entire trail show printed the trail and its changes but no merge gates, so scripts and agents lost the mergeability snapshot the repo-trail view used to print. Each open change, and the change the trail was reached through, now carries its gates: under changes[].mergeability in --json (null when not read) and as an indented block in the text view.

The lookup reuses the change-review path, generalized to any per-change read: one repository client per repository, a 20-change cap and a 5s budget, with failures summed into one stderr warning.