refactor: split cli.rs into focused submodules

Break the 1,314-line cli.rs into a cli/ directory with 6 files:
- types.rs: all structs, enums, and type aliases
- resolver.rs: SubcommandResolver and cache/env helpers
- handler.rs: VitePlusCommandHandler and VitePlusConfigLoader
- execution.rs: resolve_and_execute command variants
- help.rs: CLI help, error formatting, vitest arg helpers, and tests
- mod.rs: module declarations, re-exports, and entry points

Tightens visibility where possible (pub(super), pub(crate)) following
the patterns established in the check/ and exec/ modules.

Author: fengmk2