devex: add Playwright test writing Agent Skill (#8512)

Since there is so much ground to cover, tried to use the progressive
disclosure approach (often recommended when writing skills) such that
the agent only gets the info it needs for the given type of tests it is
writing.

Still need to try using the skill to write tests and iterate a bit from
there.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8512-devtool-add-Playwright-test-writing-Agent-Skill-2fa6d73d365081aaaf6dd186b9dcf8ce)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Amp <amp@ampcode.com>
Co-authored-by: GitHub Action <action@github.com>
Co-authored-by: Alexander Brown <drjkl@comfy.org>

browser_tests/AGENTS.md

@@ -4,11 +4,40 @@ See `@docs/guidance/playwright.md` for Playwright best practices (auto-loaded fo
 
 ## Directory Structure
 
-- `assets/` - Test data (JSON workflows, fixtures)
-- Tests use premade JSON workflows to load desired graph state
+```text
+browser_tests/
+├── assets/           - Test data (JSON workflows, images)
+├── fixtures/
+│   ├── ComfyPage.ts      - Main fixture (delegates to helpers)
+│   ├── ComfyMouse.ts     - Mouse interaction helper
+│   ├── VueNodeHelpers.ts - Vue Nodes 2.0 helpers
+│   ├── selectors.ts      - Centralized TestIds
+│   ├── components/       - Page object components
+│   │   ├── ContextMenu.ts
+│   │   ├── SettingDialog.ts
+│   │   ├── SidebarTab.ts
+│   │   └── Topbar.ts
+│   ├── helpers/          - Focused helper classes
+│   │   ├── CanvasHelper.ts
+│   │   ├── CommandHelper.ts
+│   │   ├── KeyboardHelper.ts
+│   │   ├── NodeOperationsHelper.ts
+│   │   ├── SettingsHelper.ts
+│   │   ├── WorkflowHelper.ts
+│   │   └── ...
+│   └── utils/            - Utility functions
+├── helpers/          - Test-specific utilities
+└── tests/            - Test files (*.spec.ts)
+```
 
 ## After Making Changes
 
 - Run `pnpm typecheck:browser` after modifying TypeScript files in this directory
 - Run `pnpm exec eslint browser_tests/path/to/file.ts` to lint specific files
 - Run `pnpm exec oxlint browser_tests/path/to/file.ts` to check with oxlint
+
+## Skill Documentation
+
+A Playwright test-writing skill exists at `.claude/skills/writing-playwright-tests/SKILL.md`.
+
+The skill documents **meta-level guidance only** (gotchas, anti-patterns, decision guides). It does **not** duplicate fixture APIs - agents should read the fixture code directly in `browser_tests/fixtures/`.