docs(coverage): sync docs to shipped state + wire sandcastle prompts

Closes the staleness gap after the 10-commit coverage epic shipped.

Doc sync (item 1 from the user's choice):
  - CLAUDE.md Quick Start: adds pnpm coverage:aggregate / coverage:diff
    / mutate to the command listing
  - CLAUDE.md: new "Sibling architecture: coverage (ADR-020)" section
    after the conformance gate table — captures the 4-layer table +
    points at docs/guides/coverage.md + ADR-020 + says agents must run
    coverage:diff before reporting complete
  - AGENTS.md preamble: now lists coverage as a parallel multi-latency
    quality system alongside conformance, with the same gate / latency
    framing
  - PRD frontmatter: status draft -> shipped + shipped date +
    shipping-commits list (all 10 SHAs anchoring the trace)
  - PRD findings table: each row gets a Resolution column citing the
    commit that closed it; conclusion text updated to past tense
  - ADR-020 implementation phasing: rewritten as a status table with
    each step linked to the commit that shipped it + Boot-time
    assertFeatureConformance explicitly marked Deferred with rationale
  - docs/guides/coverage.md: removed "Boot wiring lands in the next
    story" line; replaced with the deferral rationale + clarified
    that two readers (vitest, coverage:diff) consume the manifest

Sandcastle prompts (item 2 from the user's choice):
  - .sandcastle/implementer.prompt.md: new "Coverage gates" section
    after the conformance-gates list, requiring `pnpm test --coverage`,
    `pnpm coverage:aggregate`, and `pnpm coverage:diff` to all pass
    before reporting `complete`. Machine-readable JSON shape of
    coverage:diff documented (status / uncovered[] / kind enum), with
    explicit instructions on how to interpret each kind. Allowlist
    expansion requires justification + test.
  - .sandcastle/reviewer.prompt.md: AC coverage relabeled to "AC
    coverage (acceptance criteria, not test coverage)" to disambiguate;
    new check #7 "Coverage gates (ADR-020)" requiring CI's
    Coverage — diff (L1) step green + per-layer thresholds met +
    no silent allowlist expansion + manifest band drift detection.

Effect: future agent runs through sandcastle now treat coverage as a
first-class blocking gate, parallel to conformance. PRs no longer
discover coverage failures only via CI; the implementer is required
to check before reporting done, and the reviewer is required to
verify.

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

AGENTS.md

@@ -13,7 +13,12 @@ This template assumes agents (Claude, Codex, etc.) will author most feature work
 - `pnpm work dispatch --execute` — invoke sandcastle (requires `ANTHROPIC_API_KEY`)
 - `.sandcastle/` — 5 prompt templates (PRD eliciter, ADR eliciter, decomposer, implementer, reviewer); all enforce **generator-first** (`pnpm turbo gen <kind>` over hand-rolling)
 
-Every feature has a `src/feature.manifest.ts` declaring its use cases. Every `bindProductionX(ctx)` and `bindDevSeedX(ctx)` self-asserts at its tail via `assertFeatureConformance(...)`. Five conformance gates catch drift at four latency tiers: TypeScript (0s), ESLint (<1s), boot (~3s), CI (`pnpm conformance`, `pnpm fallow`). See `docs/guides/conformance-quickref.md` and `docs/guides/runbook.md` for the full workflow.
+Every feature has a `src/feature.manifest.ts` declaring its use cases AND its coverage bands. Every `bindProductionX(ctx)` and `bindDevSeedX(ctx)` self-asserts at its tail via `assertFeatureConformance(...)`. Quality is enforced by two parallel multi-latency systems:
+
+- **Conformance** (5 gates) — TypeScript brands (0s), ESLint (<1s), boot (~3s), `pnpm conformance` (~120s), `pnpm fallow` (~30–60s). Catches manifest↔code drift. See `docs/guides/conformance-quickref.md`.
+- **Coverage** (4 layers, ADR-020) — L0 vitest thresholds, L1 `pnpm coverage:diff` (cover-the-diff gate), L2 `pnpm coverage:aggregate` → committed `coverage/summary.json`, L3 `pnpm mutate` (nightly). The manifest's `coverage.bands` is the single source of truth. See `docs/guides/coverage.md`.
+
+See `docs/guides/runbook.md` for the full workflow.
 
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