docs: add convention for new assertions — prefer page objects over custom matchers (#10660)

## Summary

Add guidance to `docs/guidance/playwright.md` that new node-specific
assertions should be methods on page objects/helpers rather than new
`comfyExpect` custom matchers.

## Changes

- **What**: New "Custom Assertions" section in Playwright guidance
documenting that existing `comfyExpect` matchers are fine to use, but
new assertions should go on the page object for IntelliSense
discoverability.

## Review Focus

Documentation-only change. No code refactoring — this is a convention
for new code only.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-10660-docs-add-convention-for-new-assertions-prefer-page-objects-over-custom-matchers-3316d73d3650816d97a8fbbdc33f6b75)
by [Unito](https://www.unito.io)

---------

Co-authored-by: GitHub Action <action@github.com>

docs/guidance/playwright.md

@@ -117,6 +117,18 @@ export const test = base.extend<{
 - Keep fixtures modular — extend `@playwright/test` base, not
   `comfyPageFixture`, so they can be composed via `mergeTests`
 
+## Custom Assertions
+
+Add assertion methods directly on the page object or helper class instead of extending `comfyExpect`. Page object methods are discoverable via IntelliSense without special imports.
+
+```typescript
+// ✅ Page object assertions
+await node.expectPinned()
+await node.expectBypassed()
+
+// ❌ Do not add custom matchers to comfyExpect
+```
+
 ## Test Tags
 
 - `@mobile` — Mobile viewport tests