Adds a new section to the README detailing the collaboration workflow
using pr.pico.sh. This aims to simplify contributions by outlining
the patch request process clearly, removing barriers for new
contributors.

- Implement parse_fragment_string() to extract URL and arguments from colon-separated format
- Add build_repomix_command() to construct repomix command with parsed arguments
- Support all major repomix flags: compress, include, ignore, style, remove-comments, etc.
- Handle complex URL formats (https://, ssh://, git@) with proper colon parsing
- Add comprehensive test coverage for argument parsing and command building
- Update documentation with usage examples and supported arguments

Examples:
- repomix:https://git.sr.ht/~amolith/willow:compress
- repomix:https://git.sr.ht/~amolith/willow:include=*.py,*.md:ignore=tests/
- repomix:git@github.com:user/repo.git:compress:remove-comments

Add plugin that loads repository contents using Repomix:
- Clone git repositories to temporary directories
- Run repomix to generate AI-friendly repository summaries
- Return repomix output as a single fragment
- Clean up temporary directories automatically