refactor(verify-work): consolidate plan-fix into plan-phase --gaps

Unify gap handling - whether discovered by code verification or user
testing, gaps feed into the same plan-phase --gaps workflow.

Changes:
- Delete commands/gsd/plan-fix.md (redundant)
- Update verify-work to route to plan-phase --gaps
- Update progress Route E to detect UAT gaps
- Change UAT.md "Issues" section to "Gaps" with YAML format
- Extend plan-phase --gaps to read from both VERIFICATION.md and UAT.md
- Update diagnose-issues to output gaps in YAML format
- Update all references (debug, templates, workflows)

One path: gap discovered → diagnosed → plan-phase --gaps → fixed

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

README.md

@@ -377,7 +377,6 @@ You're never locked in. The system adapts.
 | Command | What it does |
 |---------|--------------|
 | `/gsd:verify-work [N]` | User acceptance test of phase or plan ¹ |
-| `/gsd:plan-fix [plan]` | Plan fixes for UAT issues |
 
 ### Milestones