feat: test coverage gate + plan completion audit + auto-verification

Three new gates in /ship and /review:
1. Test coverage gate: configurable thresholds (60%/80% default), hard stop
   below minimum with user override
2. Plan completion audit: discovers plan file, extracts actionable items,
   cross-references against diff, gates on NOT DONE items
3. Auto-verification: invokes /qa-only inline with plan's verification
   section, conditional on localhost reachability

Also: coverage warning in /review, plan completion data in /retro,
shared plan file discovery helper (DRY), ship metrics logging.

retro/SKILL.md.tmpl

@@ -459,6 +459,28 @@ Narrative covering:
 - If prior retro exists and has `test_health`: show delta "Test count: {last} → {now} (+{delta})"
 - If test ratio < 20%: flag as growth area — "100% test coverage is the goal. Tests make vibe coding safe."
 
+### Plan Completion
+Check review JSONL logs for plan completion data from /ship runs this period:
+
+```bash
+eval "$(~/.claude/skills/gstack/bin/gstack-slug 2>/dev/null)"
+cat ~/.gstack/projects/$SLUG/*-reviews.jsonl 2>/dev/null | grep '"skill":"ship"' | grep '"plan_items_total"' || echo "NO_PLAN_DATA"
+```
+
+If plan completion data exists within the retro time window:
+- Count branches shipped with plans (entries that have `plan_items_total` > 0)
+- Compute average completion: sum of `plan_items_done` / sum of `plan_items_total`
+- Identify most-skipped item category if data supports it
+
+Output:
+```
+Plan Completion This Period:
+  {N} branches shipped with plans
+  Average completion: {X}% ({done}/{total} items)
+```
+
+If no plan data exists, skip this section silently.
+
 ### Focus & Highlights
 (from Step 8)
 - Focus score with interpretation