feat(terraphim_agent): refactor to minimalist feature gates

[AlexMikhalev]

Summary

Refactor terraphim_agent crate to have minimalistic features:

1. Remove server feature as default - make it optional behind a server feature flag
2. Gate chat functionality behind llm feature flag - chat only available if llm flag is enabled
3. TUI (fullscreen) works offline - using TuiBackend::Local when server feature is off
4. Add firecracker feature for VM management - VM commands only available with firecracker flag

Feature Combinations

All combinations compile and pass clippy with -D warnings.

Refs #IDX