Add configurable YAML parser support for frontmatter parsing

[Copilot]

This PR implements a configurable YAML parser system for frontmatter parsing, addressing the discussion in #1 about providing flexibility for users who don't have access to the lyaml dependency.

Problem

The current implementation hardcodes the use of lyaml for parsing YAML frontmatter in markdown files. This creates a barrier for users who don't have lyaml installed or available in their environment (e.g., users without LuaRocks access).

Solution

Added a configuration system to the frontmatter module that allows users to specify custom YAML parser functions while maintaining full backward compatibility:

local frontmatter = require('codecompanion.filewise.frontmatter')

-- Option 1: Default behavior (tries lyaml, falls back to built-in parser)
-- No configuration needed - works automatically

-- Option 2: Explicit custom parser
frontmatter.setup({
  yaml_parser = function(yaml_text)
    -- Your custom YAML parsing logic
    return parsed_table_or_nil
  end
})

-- Option 3: Use the built-in parser explicitly
local yaml_parser = require('codecompanion.filewise.yaml_parser')
frontmatter.setup({
  yaml_parser = yaml_parser.parse
})

Key Features

• Backward Compatible: Existing users with lyaml see no changes in behavior
• Automatic Fallback: When lyaml is unavailable, automatically uses the built-in YAML parser
• Configurable: Users can provide their own YAML parser functions
• Zero Breaking Changes: All existing extensions continue to work without modification
• Well Documented: Added comprehensive configuration documentation to README

Implementation Details

• Modified lua/codecompanion/filewise/frontmatter.lua to support configurable parsers
• Added setup() function for configuration
• Added get_yaml_parser() function that intelligently selects the appropriate parser
• Maintained the same public API (parse() function remains unchanged)
• Updated README.md with configuration examples and usage instructions

Testing

Comprehensive testing covers all scenarios:

• Default behavior with lyaml available/unavailable
• Custom parser configuration
• Error handling and edge cases
• Integration with all three extensions (Custom Instructions, Custom Prompts, Custom Modes)
• Configuration persistence and multiple setup calls

This solution provides the requested "configuration field that allows to specify a custom YAML parser" while ensuring no disruption to existing workflows.

Warning

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Original prompt

dyamon#1 (comment)

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