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/execute-phase.md

@@ -71,15 +71,21 @@ ls -1 "$PHASE_DIR"/*-SUMMARY.md 2>/dev/null | sort
 For each plan, read frontmatter to extract:
 - `wave: N` - Execution wave (pre-computed)
 - `autonomous: true/false` - Whether plan has checkpoints
+- `gap_closure: true/false` - Whether plan closes gaps from verification/UAT
 
 Build plan inventory:
 - Plan path
 - Plan ID (e.g., "03-01")
 - Wave number
 - Autonomous flag
+- Gap closure flag
 - Completion status (SUMMARY exists = complete)
 
-Skip completed plans. If all complete, report "Phase already executed" and exit.
+**Filtering:**
+- Skip completed plans (have SUMMARY.md)
+- If `--gaps-only` flag: also skip plans where `gap_closure` is not `true`
+
+If all plans filtered out, report "No matching incomplete plans" and exit.
 </step>
 
 <step name="group_by_wave">
@@ -433,9 +439,9 @@ Present gaps and offer next command:
 
 User runs `/gsd:plan-phase {X} --gaps` which:
 1. Reads VERIFICATION.md gaps
-2. Creates additional plans (04, 05, etc.) to close gaps
-3. User then runs `/gsd:execute-phase {X}` again
-4. Execute-phase runs incomplete plans (04-05)
+2. Creates additional plans (04, 05, etc.) with `gap_closure: true` to close gaps
+3. User then runs `/gsd:execute-phase {X} --gaps-only`
+4. Execute-phase runs only gap closure plans (04-05)
 5. Verifier runs again after new plans complete
 
 User stays in control at each decision point.