feat(cli): add rm model command

[akshayatam]

Closes #59

Summary

Adds basert rm <model> to remove a locally installed model from the BaseRT model cache.

Behavior

• Removes all locally installed variants of the specified model.
• Returns a clear error when the requested model is not installed.
• Rejects unsafe model IDs before performing deletion.
• Preserves sibling models, unrelated files, and source/staging data.
• Avoids following model, variant, artifact, and sidecar symlinks during removal.
• Respects BASERT_MODELS_DIR when using a custom model cache.

Testing

Added end-to-end coverage for the rm command, including:

• successful removal of all variants for a model
• preservation of sibling and nested models
• missing and uninstalled models
• unsafe model IDs
• symlink handling
• sidecar handling
• custom model cache paths
• CLI help and required argument behavior

Also manually verified removal with multiple models installed in an isolated cache.

Validation performed:

• cargo test passes, including all 8 rm end-to-end tests
• cargo clippy passes
• cargo fmt --check reports existing formatting differences in unrelated files not modified by this PR

Documentation

Updated the CLI overview, command reference, and model management guide to document basert rm <model>.