fix: Resolve cargo fmt formatting errors and rustfmt config deprecations

[LyeZinho]

Summary

• Remove unstable rustfmt options that are only available in nightly Rust
• Replace deprecated fn_args_layout with fn_params_layout
• Fix code formatting (trailing commas in match arms, alphabetical module ordering)
• Verify with cargo fmt, clippy, and tests - all pass cleanly

Changes

• .rustfmt.toml: Removed unstable/deprecated options (comment_width, wrap_comments, fn_single_line, struct_* settings, space_* settings)
• crates/mimi-cli/src/main.rs: Add trailing commas to match arms
• crates/mimi-core/src/lib.rs: Reorder modules alphabetically (config, error, message)

Verification

• ✅ cargo fmt --check passes
• ✅ cargo clippy passes
• ✅ cargo build succeeds
• ✅ cargo test passes (5/5 tests)