Add a once exec ... command

[kevinmcconnell]

Adds a new CLI command, exec which executes a command in an application's running container. Much like docker exec, docker compose exec, etc.

• Will attach a TTY as necessary, so interactive commands work without additional flags
• Passes back the exit code from the command

Closes #59

[Copilot]

Pull request overview

Adds a new once exec CLI command that executes a command inside a running application container (similar to docker exec), including TTY attachment/resize handling and propagating the container command’s exit status back to the once process.

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Changes:

• Implement attached exec support in the Docker layer (TTY/raw mode, stdin/stdout piping, resize handling, exit-code errors).
• Add once exec <host> <command> [args...] command wiring plus argument parsing and unit tests.
• Propagate exec exit codes through the CLI entrypoint and add integration coverage for success/failure/not-running cases.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
internal/docker/container.go	Adds attached exec implementation (TTY detection, raw mode, I/O piping, resize monitoring) and exit-code error type.
internal/docker/application.go	Exposes Application.Exec and maps Docker “conflict” errors to ErrApplicationNotRunning.
internal/command/root.go	Registers the new exec subcommand and adds helpers for extracting exit codes from errors.
internal/command/exec.go	Implements the once exec Cobra command and argument splitting (including -- separator).
internal/command/exec_test.go	Unit tests for splitExecArgs behavior.
integration/docker_test.go	Integration tests validating exec exit codes and behavior when the app is stopped.
cmd/once/main.go	Exits with the underlying exec exit code when present.

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