Remove prettier-plugin-toml dependency to fix minimal template

[Copilot]

The minimal template was failing during git commit with a module not found error for prettier-plugin-toml. The error occurred because the hardcoded path to the plugin was incorrect:

[error] Cannot find module '/nix/store/lfjxr7afcn146kfb4lkm79z0c1jndamp-prettier-plugin-toml-1.0.0/lib/node_modules/prettier-plugin-toml/lib/index.js' imported from /home/samtitle/PersonalRepos/test-std-project/noop.js

This was caused by the hardcoded path ${nixpkgs.nodePackages.prettier-plugin-toml}/lib/node_modules/prettier-plugin-toml/lib/index.js in the treefmt configuration, which no longer exists due to changes in the package structure.

Following the maintainer's suggestion to drop TOML support from prettier, this PR removes the prettier-plugin-toml dependency entirely:

• Removed nixpkgs.nodePackages.prettier-plugin-toml from packages in both treefmt.nix and mdbook.nix
• Removed the --plugin option and hardcoded path from prettier configuration
• Removed *.toml from prettier's file includes list

The change preserves all other formatting functionality (nix, shell, CSS, HTML, JS, JSON, JSX, MD, MDX, SCSS, TS, YAML) while eliminating the problematic TOML formatting via prettier. TOML files will simply not be formatted automatically, but the minimal template will work without errors.

Fixes #407.

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