feat: add native flush, config, and cache commands

[youngledo]

Summary

Migrate three pure-filesystem Bash commands to Rust as Phase 1 of the complete Bash-to-Rust migration proposed in #439.

• flush: Remove and recreate tmp, var/metadata, and var/version directories with disk usage reporting. Supports qualifiers: tmp, metadata, version, or flush all by default.
• config: Open $SDKMAN_DIR/etc/config in the user's $EDITOR (defaults to vi). Supports editors with arguments (e.g., EDITOR="code --wait").
• cache: Validate that the candidates cache file at $SDKMAN_DIR/var/candidates exists and is non-empty. Displays a warning with remediation instructions if corrupt.

Context

This is the first deliverable from the RFC in #439. These three commands were chosen because:

1. They perform only local filesystem operations (no network, no shell env mutation)
2. They follow the exact same pattern as the existing 6 migrated commands
3. No architectural changes are required

Test plan

• cargo test — all 58 tests pass (11 new + 47 existing), zero regressions
• cargo clippy — zero new warnings on the three new binaries
• flush: 5 tests covering default flush, tmp-only, metadata-only, version, and empty directory
• config: 2 tests covering missing editor and successful editor launch
• cache: 4 tests covering valid cache, empty cache, missing cache, and whitespace-only cache

Refs: #439

[chloe41427]

Thanks for the PR! We appreciate the effort, but we do have contributor guidelines that we'd like all contributors to follow before opening PRs.

On the technical side: we already have a roadmap to migrate all the Bash commands to Rust, but this is sequenced after the backend work (sdkman-state migration) is complete. Once that's stable, we'll migrate the remaining subcommands in a coordinated way.

Please reach out in our Discord community if you'd like to discuss the migration roadmap or how you can help!