cli: top-level commands (up, down, ps, logs, validate, manifest)

[pierrick-fonquerne]

Expose the core orchestrator capabilities through a clap-based CLI so the developer interacts with LightShuttle through one binary.

Scope

• lightshuttle up boots the stack declared in ./lightshuttle.yml.
• lightshuttle down tears it down.
• lightshuttle ps prints a tabular view of resource statuses.
• lightshuttle logs <resource> streams logs of a single resource.
• lightshuttle validate parses and validates the manifest without starting anything.
• lightshuttle manifest dumps the resolved manifest (post-interpolation) for debugging.

Acceptance criteria

• All six commands work end-to-end on the example manifest of the spec issue.
• Exit codes documented: 0 success, 1 user error, 2 runtime error, 130 SIGINT.
• --help output reviewed for tone and clarity.
• Integration test exercises a representative subset of the commands.