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

.playwright/cli.config.json

@@ -0,0 +1,13 @@
+{
+  "browser": {
+    "browserName": "chromium",
+    "isolated": true,
+    "launchOptions": {
+      "channel": "chromium",
+      "headless": true,
+      "chromiumSandbox": true
+    }
+  },
+  "outputDir": ".artifacts/playwright-cli",
+  "outputMaxSize": 104857600
+}