Wire image cleanup endpoints into the CLI

[nfebe]

The agent now exposes auto and on-demand image cleanup (flatrun/agent#124). The CLI should let users invoke it directly and opt out of the auto behavior.

• flatrun image cleanup calls the host-wide dangling prune endpoint.
• flatrun deployment image cleanup <name> calls the per-deployment stale cleanup endpoint.
• flatrun deployment image set ... --deploy and flatrun deployment pull ... should accept --no-cleanup to skip the auto run, and a --dry-run that maps to the same field on the cleanup endpoint.

Output should show the removed image refs and freed bytes returned by the agent.