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>

docs/glossary.md

@@ -124,6 +124,27 @@ The whole-codebase auditor (`pnpm fallow`) — dead exports, unused files, dupli
 **Drift**:
 Any disagreement between a feature's manifest and its code. The conformance gates are designed to catch drift at the earliest possible latency.
 
+## Coverage
+
+**Coverage band**:
+The per-path threshold declared in `feature.manifest.ts` under `coverage.bands` — one entry per Clean Architecture layer (`entities`, `use-cases`, `controllers`) plus a `baseline` for everything else. Single source of truth read by vitest, `assertFeatureConformance`, and `pnpm coverage:diff`. See ADR-020.
+
+**L0 / L1 / L2 / L3**:
+The four coverage layers (ADR-020). **L0** = vitest per-layer thresholds (test-time). **L1** = `pnpm coverage:diff` cover-the-diff gate (post-test, CI + dispatch loop). **L2** = `pnpm coverage:aggregate` → committed `coverage/summary.json` (observability). **L3** = `pnpm mutate` Stryker mutation testing on entities + use-cases (on-demand, not default).
+
+**Diff coverage**:
+The gate that asserts every changed _executable_ line has execution count > 0 in the merged lcov. Cover-the-diff (modified + new lines both count), not cover-the-new-code. Run via `pnpm coverage:diff [<base-ref>]`.
+_Avoid:_ patch coverage, delta coverage (use "diff coverage" canonically).
+
+**Mutation testing**:
+A test-quality signal that mutates source code and re-runs tests; surviving mutants mean the test exists + executes the code but doesn't actually assert behavior. Scoped to `entities/` + `application/use-cases/` per feature. Run via `pnpm mutate [--filter @repo/<feature>]`.
+
+**Mutation score**:
+The percentage of mutants killed (i.e., caught by tests) out of all mutants generated. Per-feature threshold defaults to 80% (overridable in `feature.manifest.ts` via `coverage.mutationThreshold`).
+
+**`coverage/summary.json`**:
+The aggregated per-package + repo-level coverage snapshot, committed on merge to main. Grep-able from git history via `git log -- coverage/summary.json`. Includes timestamp + commit SHA for correlation with deploys.
+
 ## Cross-feature mechanisms
 
 **Event bus** (`IEventBus`):
@@ -232,6 +253,8 @@ The shipping rhythm. One vertical slice closes one task, becomes one PR, lands a
 - A **Channel descriptor** is public; its **Realtime handler** is always private.
 - **Brands** are attached only at **DI bind time**, by **`withSpan` / `withCapture` / `withAudit`**.
 - **Conformance** asserts the **Manifest** and code agree, at five latency tiers.
+- A **Coverage band** in the **Manifest** is read by Vitest (L0), `assertFeatureConformance` (boot), and `pnpm coverage:diff` (L1) — one decision, three enforcers.
+- **Mutation testing** is the third dimension of test quality, after "test file exists" (ESLint structural) and "test executes the code" (L0/L1 coverage).
 
 ## Flagged ambiguities
 
@@ -244,6 +267,7 @@ The shipping rhythm. One vertical slice closes one task, becomes one PR, lands a
 - **"spec"** — `docs/superpowers/specs/` design doc; never a PRD or a Jest spec file.
 - **"controller"** — one verb-noun pair per file; never a multi-method MVC controller.
 - **"module"** — avoid for packages (use "package"); reserve for Node ESM/CJS module semantics if needed.
+- **"coverage"** — qualify when ambiguous: **L0 coverage** (per-layer thresholds, test-time) | **diff coverage / L1** (cover-the-diff gate) | **aggregate coverage / L2** (committed summary.json) | **mutation coverage / L3** (Stryker mutation score, not line coverage).
 
 ## Cross-references