feat(retro): absorb inline git/awk metrics into bin/gstack-retro-metrics + carve report format

RETRO_METRICS_PROTO: 1 contract, local git reads only (fetch stays in the
skill prose), degraded path documented in the skeleton.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

retro/sections/manifest.json

@@ -0,0 +1,14 @@
+{
+  "$schema": "https://gstack.dev/schemas/section-manifest.json",
+  "skill": "retro",
+  "version": 1,
+  "note": "PASSIVE registry (v2 plan T9 / CM2). id/file/title/trigger text ONLY. The skeleton's decision-tree prose decides WHEN to read. No machine predicate here.",
+  "sections": [
+    {
+      "id": "report-format",
+      "file": "report-format.md",
+      "title": "Narrative report structure — tweetable summary, section-by-section output template, Test Health, Plan Completion, personal deep-dive, team breakdown, wins/improvements/habits (Step 14)",
+      "trigger": "writing the retrospective narrative (Step 14, after all metrics are computed and compared)"
+    }
+  ]
+}