feat(cache): add `wmk cache` CLI command group (list, clear, info)

Summary

The cache module (modelkit/cache/) has well-designed primitives (list_cached_models(), get_cache_dir(), etc.) but no CLI entrypoint. Users cannot inspect or manage the cache without manually navigating ~/.cache/winml/.

Motivation

• list_cached_models() is exported and tested but completely unreachable from the CLI
• Users need visibility into what's cached, how much space it uses, and the ability to clean up
• Common developer workflow: export → inspect cache → clear stale artifacts

Proposed Subcommands

wmk cache list

• Uses list_cached_models() directly
• Groups by model slug, shows task/stage/size
• Rich Table output (default) or JSON (--format json)
• --model filter by model slug

wmk cache clear

• wmk cache clear — clear all (with confirmation prompt)
• wmk cache clear --model <slug> — clear specific model
• --yes flag to skip confirmation
• Shows what was deleted and freed space

wmk cache info

• Shows cache directory path
• Total size, number of models, number of artifacts
• Environment variable status (WMK_CACHE_DIR)

Implementation Plan

Patterns to Reuse

• Rich Table pattern from modelkit/commands/sys.py
• Click CliRunner from existing test files
• click.confirm() for deletion confirmation
• Auto-discovery in cli.py for command registration

Acceptance Criteria

• wmk cache list displays cached models in table format
• wmk cache list --format json outputs JSON
• wmk cache list --model <slug> filters output
• wmk cache clear --yes removes all cache artifacts
• wmk cache clear --model <slug> --yes removes specific model
• wmk cache info shows cache path, size, and model count
• All new code has pytest coverage
• uv run ruff check passes on all modified files