docs(prds): seed agent-first coverage architecture PRD

Captures the brainstorm convergence (2026-05-13) into a draft PRD that
seeds the implementation epic. Decisions captured:

- 4-layer coverage architecture (L0 vitest thresholds, L1 diff coverage,
  L2 aggregate summary.json, L3 mutation testing on entities + use-cases)
- Manifest-driven coverage band as the single source of truth — vitest
  config, assertFeatureConformance boot check, and pnpm coverage:diff
  all derive expectations from feature.manifest.ts
- Cover-the-diff (every changed line exercised) not cover-the-new-code
- Aggregate trend committed as coverage/summary.json; no SaaS dep
- Stryker for mutation, on-demand only, NOT in default pnpm test

L0 verification findings (also captured in the PRD):
- auth / blog / marketing-pages: green (100% layer bands hold)
- navigation: real test gaps in entities + controllers (~86%)
- media: missing @vitest/coverage-v8 dep + vitest.config has no
  coverage block at all (entirely absent — the most extreme case of
  the duplication problem the manifest-driven keystone eliminates)

Status remains draft until the human review pass. Decomposer will
refuse to run on draft.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/work/_state.json

@@ -1,5 +1,5 @@
 {
-  "updated_at": "2026-05-13T08:18:31.249Z",
+  "updated_at": "2026-05-13T11:37:22.767Z",
   "epics": {
     "template-reset-v1": {
       "status": "done",