feat(cli): add a studio subcommand behind the studio feature

[LeadcodeDev]

cargo install --features studio built the studio but left it reachable only as a second binary, so the first name anyone tries — rustmotion studio — answered unrecognized subcommand. That is what a user reported.

rustmotion studio now opens it, taking the same -f / -d as rustmotion-studio. The standalone binary is unchanged and still installed by the same feature; removing it would break anyone who scripted it, and it costs four lines.

The variant is compiled either way, and #[cfg_attr(not(feature = "studio"), command(hide = true))] keeps it out of --help when the feature is off, which is what CLAUDE.md asks for. Typing it anyway then gets:

Error: this build of rustmotion has no studio: it is behind the `studio` feature, which is off by default because it pulls gpui and a native GUI toolchain. Reinstall with it on:
  cargo install --git https://github.com/LeadcodeDev/rustmotion --features studio

Gating the variant out entirely was the other option and was rejected: it produces unrecognized subcommand 'studio', which is the message that caused the report. A hidden subcommand advertises nothing and still explains itself.

studio::run called Cli::parse() on the process argv, which would have failed on the studio token, so its body moved to run_with(file, dir) and both entrypoints call that. studio::command() had no caller anywhere in the repo and was removed.

Verification — tests/studio_subcommand.rs drives the built binary and splits on the feature: with it, studio is in the subcommand list and studio --help carries --file and --dir; without it, no studio line in --help, and rustmotion studio exits non-zero naming --features studio and not unrecognized subcommand. Run both ways, plus cargo fmt --all --check, cargo clippy --workspace --all-targets with and without --features rustmotion/studio, and cargo test --workspace --features rustmotion/studio (0 failed). CI only runs clippy with the feature on, so the feature-off branch was checked locally.

Closes #458