uninstall: --keep-* flags are ignored when config, data, and cache directories overlap (default layout) #626

Problem

On a default install, rocm uninstall ignores a --keep-* flag when the config, data, and cache directories overlap.

By default, default_config_dir() and default_data_dir() both resolve to ~/.rocm, and default_cache_dir() resolves to ~/.rocm/cache (crates/rocm-core/src/runtime.rs). build_uninstall_plan() (apps/rocm/src/main.rs) plans one removal per category unless that category's flag is set, so:

• --keep-data still plans config: ~/.rocm, which deletes the downloaded models, managed ROCm SDK installs, and uv-cache that the flag is meant to keep.
• --keep-config still plans data: ~/.rocm, which contains the config.
• --keep-cache still plans config: ~/.rocm, which contains cache/.

A user who passes --keep-data on a default install loses every managed runtime and downloaded model.

Suggested fix

When planning removals, skip any directory that equals or contains a directory of a kept category (and, for a kept category inside a removed one, remove only the non-kept entries, or refuse and explain). Add a dry-run test on the default layout for each --keep-* flag.

Current mitigation

The README and docs/engine-plugins.md now tell users to pass --keep-config together with --keep-data on a default install (#616). Remove that caveat when this is fixed.

Found by the automated review on #616.