feat(composer): add provider slash commands and skills

[tyulyukov]

Summary

• Added provider slash commands and skills to the composer command menu, with search filtering
• Refactored Claude capability probing to use async generators instead of maxTurns for cleaner initialization
• Moved additional directories handling to the bootstrap phase (sent with thread.meta.update on thread creation)
• Fixed DOM connectivity checks in useSmoothReveal to prevent errors when spans are unmounted mid-animation
• Added debug logging for provider initialization data (commands and skills counts)
• Simplified additional directories computation in ClaudeAdapter

Testing

• Provider slash commands appear in composer menu when provider is selected
• Provider skills appear in composer menu with /skill trigger
• Slash commands and skills can be selected and insert correct text replacement
• Additional directories are sent to server in bootstrap createThread phase
• Debug logs show correct command/skill counts on provider initialization
• Smooth reveal animation doesn't break when DOM elements are removed
• Existing model selection and command menu filtering still works
• No regressions in thread creation or message sending