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>

agents/gsd-planner.md

@@ -1296,7 +1296,7 @@ Execute: `/gsd:execute-phase {phase}`
 
 ### Next Steps
 
-Execute: `/gsd:execute-phase {phase}`
+Execute: `/gsd:execute-phase {phase} --gaps-only`
 ```
 
 ## Revision Complete