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>

commands/gsd/execute-phase.md

@@ -1,7 +1,7 @@
 ---
 name: gsd:execute-phase
 description: Execute all plans in a phase with wave-based parallelization
-argument-hint: "<phase-number>"
+argument-hint: "<phase-number> [--gaps-only]"
 allowed-tools:
   - Read
   - Write
@@ -30,6 +30,9 @@ Context budget: ~15% orchestrator, 100% fresh per subagent.
 <context>
 Phase: $ARGUMENTS
 
+**Flags:**
+- `--gaps-only` — Execute only gap closure plans (plans with `gap_closure: true` in frontmatter). Use after verify-work creates fix plans.
+
 @.planning/ROADMAP.md
 @.planning/STATE.md
 </context>
@@ -43,6 +46,7 @@ Phase: $ARGUMENTS
 2. **Discover plans**
    - List all *-PLAN.md files in phase directory
    - Check which have *-SUMMARY.md (already complete)
+   - If `--gaps-only`: filter to only plans with `gap_closure: true`
    - Build list of incomplete plans
 
 3. **Group by wave**

commands/gsd/verify-work.md

@@ -156,7 +156,7 @@ Fix plans verified ✓
 
 **Execute fix plans** — run diagnosed fixes
 
-/gsd:execute-phase {Z}
+/gsd:execute-phase {Z} --gaps-only
 
 <sub>/clear first → fresh context window</sub>