build(web): replace Playwright MCP with project-scoped Playwright CLI

- feat: add `.agents/skills/playwright-cli/` with complete Skill
  markdown, agent interface, and 9 reference files covering session
  management, spec-driven testing, video recording, tracing, storage,
  request mocking, element attributes, test debugging, and running
  custom code
- feat: add `.playwright/cli.config.json` to configure the Playwright
  CLI
- feat: add `web/package.json` `playwright:cli` script that resolves
  `playwright cli` from the repo root
- build: remove `@playwright/mcp` dependency from `web/package.json`
- build: set `XDG_CACHE_HOME` in `mise.toml`
- build: delete `.codex/config.toml` (MCP server config)
- docs: update `README.md`, `docs/web/roadmap.md`,
  `docs/web/decisions.md`, and `web/README.md` to reflect the new
  Playwright CLI approach

README.md

@@ -42,7 +42,9 @@ are available while their APIs and large-file behavior continue to evolve.
 
 The frontend uses React + TypeScript and communicates with the backend via HTTP
 API. Its first milestone supports login, a root file list, single-file upload,
-and authenticated download. Frontend source is maintained in `web/`.
+and authenticated download. Frontend source is maintained in `web/`; see
+[`web/README.md`](web/README.md) for development, testing, and browser-debugging
+instructions.
 
 ---