docs: require a snapshot test for transcript/UX-affecting changes

Documents the snapshot tier and makes its use a convention. AGENTS.md gains the
test:snapshot / test:snapshot:record commands (and corrects the now-stale
`pnpm run test` include comment — the unit suite picks up examples/*/tests too),
plus a Conventions bullet: a change affecting the editor-facing transcript or
end-to-end agent UX needs a snapshot test (or an explicit note why none
applies); a pure internal refactor is exempt. The dsh-code-review skill gains a
matching reviewer-only check — review the golden diff itself, since a changed
*.golden is a behavior change in disguise.

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

@@ -43,6 +43,7 @@ Where your independent reasoning earns its keep. Start here, then keep going acr
 - **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.
+- **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")?
 
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