Release Twitter digest and MCP skills v1.5.12

[boboliu-1010]

Summary

• split and ship the twitter-digest and twitter-mcp skills
• add curl-based installers/uninstallers for Codex and Claude Code
• keep digest source selection isolated between API and browser flows
• publish v1.5.12 release install docs and cleanup behavior

Testing

• python3 -m py_compile twitter-digest/scripts/*.py
• sh -n twitter-digest/install.sh twitter-digest/uninstall.sh twitter-mcp/install.sh twitter-mcp/uninstall.sh
• verified digest source selection cases: no API -> browser, saved token -> API, refresh config -> API
• verified uninstall dry-run cleanup for skill backups, xurl app/config, MCP registration, and xurl CLI