docs(dsh-code-review): test sufficiency — real usage, not just 100% coverage

Expand the "Test quality" reviewer check: 100% coverage proves lines ran, not
that the feature works the way it ships. Judge sufficiency on two axes —
would the test fail on a regression, and does it exercise the REAL thing
(genuine collaborator, real entry path, verify the world) rather than faking
inputs just enough to cover every line. Call out the specific trap of a
happy-path test that hits a line whose PURPOSE is a mid-flight/error/recovery
scenario it never actually drives — the exact gap a clean-turn "durability"
test would miss.

.agents/skills/dsh-code-review/SKILL.md

@@ -42,7 +42,7 @@ Where your independent reasoning earns its keep. Start here, then keep going acr
 - **e2e verifies the world, not the agent's self-report.** For real-API tests, confirm the assertion re-runs the command/checks the file externally — a keyword probe lets a cheating agent pass (see AGENTS.md e2e bullet). For a behavior change to the agent's real flows, a no-key/mock test alone is usually insufficient: a with-key e2e (especially a smoke test that boots the real example and checks the world) is cheap here and catches "green units, broken product" — encourage it rather than treating real-API tests as expensive (see AGENTS.md § Secrets / .env).
 - **Plugin export shape + real-loader coverage.** A new/changed `cordis.yml`-loaded plugin: is it a function/namespace plugin (`name`/`inject`/`Config`/`apply` named exports) with NO `export default`? A stray default export makes the Loader's `unwrapExports` drop `inject` and the plugin crashes at load with `cannot get property … without inject` — invisible to hand-built `ctx.plugin({...})` tests and to line coverage. Confirm there's a test driving it through the REAL loader path (the no-key subprocess e2e for ACP is the model). And any opportunistic read of a service NOT in `static inject` should use `ctx.get(name)`, not `ctx.<name>` (the property proxy throws through a foreign shadow). See [packages/AGENTS.md](../../../packages/AGENTS.md) and [docs/postmortem/0001](../../../docs/postmortem/0001-acp-default-export-drops-inject.md).
 - **Seam discipline.** New swappable capability? Check it's split per the capability-seams RFC (interface / impl / consumer), and that the consumer injects the interface key, never an implementation type.
-- **Test quality.** A test that passes but asserts the wrong thing is worse than none. Check that new tests would actually fail if the behavior regressed, and that they exercise the contract (events fired, disposal reached) rather than restating the implementation.
+- **Test quality — sufficiency, not just coverage.** 100% per-file coverage and a green suite are necessary, not sufficient: they prove the lines *ran*, not that the feature *works the way it ships*. Judge whether the tests are sufficient on two axes. (1) **Would they fail if the behavior regressed?** A test that passes but asserts the wrong thing — or restates the implementation instead of the contract (events fired, disposal reached, the world changed) — is worse than none. (2) **Do they exercise the REAL thing, the way it's actually used?** Prefer the genuine collaborator over a fake, drive the change through its real entry path (the cordis Loader, the ACP bridge, a booted subprocess — not a hand-built `ctx.plugin({...})` that bypasses `unwrapExports`), and verify the WORLD (re-read the file/log/registry externally), not the agent's self-report. A test that fakes the inputs just enough to cover every line will agree with whatever the author assumed; the real thing won't. When a test sets up a *clean/happy* path to reach a line, ask whether the line's PURPOSE is exercised — e.g. a durability/teardown path "tested" by a fully-completed turn never proves the mid-flight teardown it exists for; a torn-tail recovery branch covered by a well-formed log never proves recovery. Flag tests that hit the line but not the scenario. See AGENTS.md § Defensive patterns "Line coverage is not behavior coverage" and "Prefer the REAL implementation over a mock/stand-in in tests".
 - **Snapshot coverage for transcript/UX changes.** If the PR changes the editor-facing transcript or end-to-end agent UX — the ACP bridge's event→update translation, the agent loop's observable output, tool presentation, or anything an editor renders — it must add or update a snapshot scenario (`examples/*/tests/**/*.snapshot.ts`, goldens under `examples/acp-agent/tests/snapshots/`) or note explicitly why none applies (AGENTS.md § Conventions). Review the golden diff itself: a changed `stdout.golden.txt` / `session.golden.txt` is a behavior change in disguise — confirm it's intended, not an accidental regression someone re-recorded away. A pure internal refactor with no observable-output change is exempt, but the PR should say so. See [docs/rfc/implemented/2026-06-19-acp-snapshot-tests.md](../../../docs/rfc/implemented/2026-06-19-acp-snapshot-tests.md).
 - **Intent and contracts.** Does the change do what the PR says, and honor the documented contract on *both* sides of every seam it touches (see AGENTS.md "Honor cross-seam contracts on BOTH sides")?