feat(ai-integration): API-coverage verify:pre gate (#1562)

Full API Coverage by Default — Opt Out, Never Opt In. A phase that integrates
an external API/SDK/service can no longer seal without a decided coverage matrix.

- src/api-coverage.cts: deterministic detector (compound verb+noun signal +
  <Service> API/SDK surface; stopword-guarded; strips fenced code) + matrix
  parse/validate/render with field-length caps.
- check api-coverage.verify-pre: blocking seal-time gate; phase arg resolved as
  a token under .planning/phases/ only (traversal-neutralized); validates
  COVERAGE.md or blocks iff a strong integration signal is detected and no
  matrix exists; fail-closed when phases tree exists but phase unresolvable.
- capabilities/ai-integration: workflow.api_coverage_gate config key (default
  true), plan:pre contribution, blocking verify:pre gate. Data-driven.
- gsd-core/workflows/verify-work.md: generic verify:pre gate dispatch.
- Tests: detector FP/FN + matrix validation + fast-check bijection; gate e2e.
  Code+security review findings fixed (stopword FP, scope containment, pipe/cap
  rejection, prompt-injection message hygiene).
- Regenerated registry/matrix/loop-host-contract/goldens/baseline + docs.

Closes #1562

.changeset/1562-api-coverage-gate.md

@@ -0,0 +1,5 @@
+---
+type: Added
+pr: 2065
+---
+**Phases that integrate an external API/SDK/service can no longer seal without a decided coverage matrix** — a new `api-coverage` gate on the `ai-integration` capability blocks `/gsd:verify-work` until the phase produces a `COVERAGE.md` enumerating the API's full capability surface, with every non-integrated capability an explicit, reasoned opt-out. Full coverage is the default; the matrix is the subtraction record, so "we integrated the API" can no longer silently mean "we integrated whatever the first use case exercised." Toggleable via `workflow.api_coverage_gate` (on by default). (#1562)