[FEATURE] `delete` command

[CarloQuick]

Feature Summary

Add a delete command that removes a container's filesystem and manifest entry — the inverse of create. Supports a --force flag to kill running containers before deletion.

cargo run -- delete busybox-container
cargo run -- delete busybox-container --force

Problem/Use Case

There's no way to clean up containers after they're created and stopped. Users have to manually delete directories and edit container_manifest.json. Every container runtime needs a way to remove containers.

Proposed Solution

delete removes the container directory under {BENTO_CONTAINERS_PATH}/{name}/ and its entry in container_manifest.json.

Default behavior:

• Deletes containers in Created or Stopped state
• Refuses to delete Running containers

With --force:

• Kills running containers first (SIGKILL), then deletes

Container not found → bail, same as other commands.

Does NOT touch the image directory — images are shared across containers.

Acceptance Criteria

• Feature is implemented and working
• Tests are written and passing
• Documentation is updated
• Performance impact is acceptable

Priority

Medium

Implementation Details

Files to touch: bento_cli.rs, main.rs, runtime.rs

Things to be aware of:

• rollback_container_manifest() and rollback_dirs() already exist and do most of the heavy lifting
• After --force killing a running container, there's a race with the parent process in fork_into_namespaces doing its waitpid → status update → unmount_and_clean_up. Look at how stop handles this timing (the sleep + poll pattern around line 527-549 of runtime.rs)
• Handle the case where the container directory is already gone (someone manually deleted it) — still clean the manifest
• Handle the case where manifest says Running but pid is None — --force should still clean up even if kill_container() fails

UX messages:

• 🍱 Container {name} deleted
• 🍱 Container {name} killed and deleted
• Container '{name}' is currently running. Use --force to kill and delete it.