Support `.config/` directory convention and additional config formats (YAML, TOML)

Description

Repomix should support the increasingly common .config/ directory convention for configuration files, and expand format support beyond TypeScript, JavaScript, and JSON.

Summary

Many tools in the JavaScript ecosystem now support placing config files in a .config/ directory to reduce root-level clutter (e.g., Nuxt, ESLint flat config, Vitest). Repomix currently only searches the project root for repomix.config.{ts,js,json}.

Additionally, YAML and TOML are popular config formats that Repomix doesn't currently support. Projects that prefer these formats have to use JSON or JS instead.

Proposed behavior

Config discovery order:

1. Explicit --config <path> (if provided)
2. Project root: repomix.config.{ext}
3. .config/ directory: .config/repomix.{ext} or .config/repomix.config.{ext}
4. Global config directory (fallback)

New supported formats:

• YAML (.yaml, .yml)
• TOML (.toml)
• JSONC (.jsonc) and JSON5 (.json5) — already partially supported, now first-class

repomix --init improvements:

• Interactive prompt to choose config file location (project root vs .config/ directory)

Potential Approach

Replace the manual config file discovery and loading logic with unjs/c12, the config loader used by Nuxt and the unjs ecosystem. c12 handles file discovery across directories, dynamic TS/JS loading, and multi-format parsing out of the box.

Future possibilities

c12 also supports features that could be enabled later:

• RC files (.repomixrc)
• Config in package.json ("repomix" key)
• Config inheritance/extending (extends field)