feat(browser): W4-C BrowserProvider contract and camofox-browser adapter skeleton

- contracts/browser.ts: provider kind, per-capability {supported, verified} flags,
  session/tab/control-lease rows, NavigationBudget, redacted BrowserSnapshot,
  DownloadArtifact with blob/secret refs, BrowserErrorKind
- adapters/browser: BrowserProvider interface, camofox client (health, capabilities
  static + /openapi.json probe, openSession/navigate/snapshot/screenshot; rest
  not_implemented for M2), none provider, redactSnapshot, profile serial lock,
  domain allowlist budget; Bearer access key via BrowserSecretResolver
- testkit/browser: mock camofox server (auth, fixtures with secrets, faults)
- db 0023: browser_provider/session/tab/control_lease with one-active-lease and
  one-live-session-per-profile partial unique indexes; types regenerated
- docs/design/browser-provider.md: contract, camofox OpenAPI 1.17.0 mapping,
  unverified items and M2 prototype checklist; Playwright is not a product browser

Co-Authored-By: Claude Opus 5.5 <[email protected]>

packages/adapters/src/index.ts

@@ -1,5 +1,6 @@
 // @umt/adapters 公开入口;W1 起在此导出实现
 export const PACKAGE_NAME = "@umt/adapters";
+export * from "./browser/index.ts";
 export * from "./emby/index.ts";
 export * from "./fs/index.ts";
 export * from "./qbittorrent/index.ts";