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>

.sandcastle/reviewer.prompt.md

@@ -26,12 +26,16 @@ If you suspect the implementer hand-rolled what should have been generator outpu
 
 ## Your checks
 
-1. **AC coverage**: every checkbox in the task's AC list is verifiably satisfied by the diff. Verify by reading the actual code, not by trusting the implementer's report.
+1. **AC coverage** (acceptance criteria, not test coverage): every checkbox in the task's AC list is verifiably satisfied by the diff. Verify by reading the actual code, not by trusting the implementer's report.
 2. **Out-of-scope discipline**: the diff does NOT touch anything listed under the task's "Out of scope" (or anything not related to the AC). Over-engineering / drive-by refactors are rejection causes.
 3. **Manifest-first ordering**: if a new use case landed, the manifest was updated; tests exist; the factory was wrapped at bind time.
 4. **Conformance gates**: the diff's tests + lint + typecheck pass. (You don't run them yourself; sandcastle's CI step does. Trust the CI status, reject if it's red.)
 5. **Generator-first**: see the section above. Hand-rolled code that should have been generated is a rejection.
 6. **Fallow audit**: verify the implementer ran `pnpm fallow:audit` and it passed. If their diff increases dead exports / dupes / circular deps / complexity beyond the baseline, that's a rejection cause unless the implementer's notes explicitly justify it.
+7. **Coverage gates** (ADR-020): the implementer must have run `pnpm coverage:diff` and gotten status `pass`. The CI surfaces this as the "Coverage — diff (L1)" step; if it's red, reject. Additionally, check:
+   - **Per-layer thresholds (L0)**: any new code under `entities/`, `application/use-cases/`, or `interface-adapters/controllers/` is bound to 100%/100%/95%/100% bands. If the test run produced threshold errors, that's a rejection.
+   - **No silent allowlist expansion**: if `scripts/coverage/diff.mjs`'s `ALLOWED_GLOBS` grew, the implementer's notes must explain why (and the matching test fixture must exist in `scripts/coverage/diff.test.mjs`).
+   - **Manifest coverage band drift**: if `feature.manifest.ts` was edited, its `coverage:` section must match `DEFAULT_COVERAGE_BANDS` from `@repo/core-shared/conformance/coverage` (or carry an explicit override the implementer's notes justify).
 
 ## Output format