ci(storybook): wire test:storybook into CI (#1251) (#1297)

* ci(storybook): wire test:storybook into CI (#1251)

- Add test:storybook:watch to clients/web for interactive iteration; the
  one-shot script already existed.
- Surface test:storybook at the repo root, delegating to the web client.
- Add a separate CI step that installs the Playwright Chromium binary
  and runs the storybook play-function suite after validate. Kept out of
  validate because the suite needs a real browser and is ~10s slower
  than the unit tests.
- Document the new pre-push step in AGENTS.md.

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

* ci(storybook): cache Playwright browsers + tighten AGENTS wording

Cache the Chromium binary keyed on clients/web/package-lock.json so the
~150MB download is a no-op on CI cache hits. Also clarify that CI runs
test:storybook after the unit/lint/build checks (not literal `validate`,
which is a local-only script).

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

---------

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

clients/web/package.json

@@ -16,7 +16,8 @@
     "test": "vitest run --project=unit",
     "test:watch": "vitest --project=unit",
     "test:coverage": "vitest run --project=unit --coverage",
-    "test:storybook": "vitest run --project=storybook"
+    "test:storybook": "vitest run --project=storybook",
+    "test:storybook:watch": "vitest --project=storybook"
   },
   "dependencies": {
     "@emotion/react": "^11.14.0",