merge: upgrade to upstream v0.50.95 + keep custom additions
Upstream v0.50.95 features merged (Russian localization, slash commands,
mic toggle fix, gateway sync fix, KaTeX/Prism.js, etc.)
Custom additions preserved:
- Tier-2 agent switching commands in commands.js
- MC panel in index.html + MC CSS
- _resolve_cli_toolsets() in config.py
- Custom routes.py, server.py, boot.js, i18n.js, messages.js, workspace.js
Files with conflict resolution (took upstream, custom code in other files):
- CHANGELOG.md, config.py, commands.js, index.html, panels.js, style.css, ui.js