Harden delete-mode validation and registry startup behavior

[junwha]

Motivation

• Fix a startup failure when running the registry with Podman if the host bind-mount path specified by DDIFF_REGISTRY_VOLUME does not exist.
• Make non-interactive registry startup more robust in CI-like or restricted environments where -it and Podman networking may fail.
• Add first-class support for deleting manifests (explicit delete command and --delete on load) and provide an integration test that validates delete + GC flows.

Description

• Ensure the host registry volume path is created before launching the registry by adding Path(ddiff_register_volume).mkdir(parents=True, exist_ok=True) inside run_registry() and remove -it from the run invocation to avoid interactive flags in non-TTY runs.
• Add _request_manifest_digest() and delete_image() functions to resolve a manifest digest, issue an HTTP DELETE to the registry, and trigger registry garbage-collect via podman/docker exec.
• Add --delete handling to load (load_image(..., delete_after_load=True)) and register a new top-level delete command for explicit deletion.
• Add test/test_delete_mode.sh which builds local images, starts a local registry with delete API enabled, generates a diff archive, verifies ddiff delete <tag>, and verifies ddiff load --delete removes the manifest; the script also improves registry image detection and pre-creates registry volume dirs.
• Update README.md to document the server delete behaviour and the load/delete usage.

Testing

• Ran python3 -m py_compile ddiff.py which completed successfully.
• Ran syntax check bash -n test/test_delete_mode.sh which passed.
• Ran ./test/test_delete_mode.sh in this environment which exercises the updated flows but exited with SKIP because Podman networking namespace setup is not permitted here (setns: IO error: Operation not permitted), and the test correctly reports SKIP by design rather than a failure.

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Codex Task