feat: add mino exec command for running commands in active containers

[dean0x]

Summary

There is no way to exec into a running container. This is a standard container workflow — users debugging their sandbox need it.

Proposed UX

mino exec                    # exec into most recent active session with default shell
mino exec <session-id>       # exec into specific session
mino exec -- ls -la          # run specific command in most recent session

Acceptance Criteria

• mino exec subcommand added to CLI
• Resolves session → container ID
• Delegates to podman exec -it <container> <command>
• Works on both Linux (native podman) and macOS (OrbStack)
• Error with helpful hint if no active session exists

[dean0x]

Implemented in #42 (v1.4.0)