feat(execute-phase): add --gaps-only flag for gap closure execution

After verify-work diagnoses issues and creates fix plans, the next
command now explicitly signals intent with --gaps-only. This eliminates
redundant state discovery where execute-phase had to figure out why it
was being asked to run on an already-complete phase.

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

get-shit-done/workflows/verify-work.md

@@ -505,7 +505,7 @@ Plans verified and ready for execution.
 
 **Execute fixes** — run fix plans
 
-`/clear` then `/gsd:execute-phase {phase}`
+`/clear` then `/gsd:execute-phase {phase} --gaps-only`
 
 ───────────────────────────────────────────────────────────────
 ```
@@ -559,5 +559,5 @@ Default to **major** if unclear. User can correct if needed.
 - [ ] If issues: gsd-planner creates fix plans (gap_closure mode)
 - [ ] If issues: gsd-plan-checker verifies fix plans
 - [ ] If issues: revision loop until plans pass (max 3 iterations)
-- [ ] Ready for `/gsd:execute-phase` when complete
+- [ ] Ready for `/gsd:execute-phase --gaps-only` when complete
 </success_criteria>