refactor(cli): add runtime port seam

[2233admin]

Summary

• Add a small runtime port layer for filesystem, command, platform, and sidecar operations.
• Move Tauri-specific calls behind the runtime adapter.
• Keep existing GUI behavior unchanged while making non-GUI CLI entrypoints possible.

Why

This is the first step toward a headless NetMount CLI that automation tools and coding agents can call without launching the GUI or reading internal config files directly.

Test Plan

• pnpm run typecheck
• pnpm run lint
• pnpm exec vitest run