docs(coverage): ADR-020 + glossary entries + hook keyword group

Architecture record for the agent-first coverage initiative seeded by
the 2026-05-13 PRD. Captures the durable decisions:

- 4-layer architecture (L0 vitest, L1 diff, L2 aggregate, L3 mutation)
- Manifest-driven coverage band as single source of truth (vitest +
  assertFeatureConformance + pnpm coverage:diff all read from it)
- Cover-the-diff (changed lines), not cover-the-new-code
- Committed coverage/summary.json (no SaaS), trend via git log
- Mutation testing scoped to entities + use-cases, on-demand only
- Machine-first output format (JSON stdout, human stderr)

Glossary gets a new "Coverage" section with 7 entries (coverage band,
L0-L3 layers, diff coverage, mutation testing, mutation score,
coverage/summary.json), plus two relationship rows and a flagged
ambiguity for "coverage" qualifiers.

prompt-context.sh hook gets a 9th keyword group — when a prompt
mentions coverage / uncovered / lcov / mutation / stryker, the
relevant ADR + guide path are injected as additional context for
the turn.

This is the documentation layer of the coverage epic. Implementation
(manifest schema, vitest auto-derive, scripts, boot assertion,
mutation tooling) lands in subsequent stories.

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

.claude/hooks/prompt-context.sh

@@ -34,6 +34,9 @@ fi
 if echo "$prompt" | grep -qE 'boundary|boundaries|cross-package|cross feature'; then
   inject+=('Boundaries: ADR-006 + ADR-010. Five tags (app|core|core-composition|feature|tooling). Features may only depend on core + tooling. Enforced by ESLint + Turborepo boundaries.')
 fi
+if echo "$prompt" | grep -qE 'coverage|uncovered|lcov|mutation|stryker|coverage band'; then
+  inject+=('Coverage: ADR-020 + docs/guides/coverage.md (cookbook). 4 layers — L0 vitest thresholds, L1 pnpm coverage:diff (cover-the-diff), L2 coverage/summary.json (committed trend), L3 pnpm mutate (Stryker on entities + use-cases). Manifest-driven: feature.manifest.ts coverage.bands is the single source of truth.')
+fi
 
 if [ ${#inject[@]} -gt 0 ]; then
   echo "=== context-relevant pointers (from .claude/hooks/prompt-context.sh) ==="