Refactor sandbox shell into modular package

[lydakis]

Summary

• turn the sandfs.shell module into a package that focuses SandboxShell on dispatching, command registration, and visibility enforcement while the new command registry handles loading handlers
• move host materialization helpers into sandfs/shell/host.py and update the dispatcher to use the shared helper
• split the built-in commands across dedicated modules (navigation, file ops, text/search, python, host, etc.) that self-register via the registry so new commands no longer require touching the core class

Testing

• uv run pytest

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Codex Task