feat(auth): add site login and whoami commands

[jackwener]

Summary

Adds agent-discoverable site auth commands:

• opencli <site> whoami: verify the current browser session and return the logged-in identity; never opens the login page.
• opencli <site> login: verify first; if not logged in, open the site login page in a foreground browser window and poll until the human completes login.

Initial sites:

• twitter
• bilibili
• xiaohongshu
• douyin
• github

Design Boundaries

• No login --check; whoami is the check verb.
• No logout.
• No multi-account / --as.
• No credential autofill, CAPTCHA, 2FA, or passkey automation. Human authenticates; OpenCLI only opens and verifies.
• No sitemap retrofit in this PR.

Implementation Notes

• Adds shared registerSiteAuthCommands(...) helper under clis/_shared/site-auth.js.
• Each site has a thin auth.js that registers real login and whoami adapter commands, so help/list/completion/manifest all see them normally.
• Adds defaultWindowMode command metadata so login opens foreground by default while other browser commands remain background by default.
• Polling uses low-impact auth cookie checks first, then runs the identity probe once auth appears.

Verification

• npx vitest run --project adapter clis/_shared/site-auth.test.js
• npx vitest run --project unit src/execution.test.ts -t "defaultWindowMode|browser common options"
• npm run typecheck
• npm run build
• npm run docs:build
• git diff --check
• CLI smoke: node dist/src/main.js github --help, node dist/src/main.js twitter login --help, manifest assertion for 10 auth commands and foreground login default