Add Rust/Cargo target directory cleanup (project-level)

[bysiber]

When building Rust projects, the target/ directory in each project can grow to 5-20 GB. ClearDisk already supports ~/.cargo/registry but could also scan for project-level target/ directories similar to how it finds node_modules.\n\nCache Details:\n- Location: <project>/target/\n- Typical size: 5-20 GB per Rust project\n- Safe to delete: Yes (rebuilt on next cargo build)\n\nThis is a good first issue for Rust developers who want to contribute.