feat: support external custom instruction files and extension-based targeting

[roomote]

Summary

This PR implements two enhancements to improve the flexibility and scalability of RooCode's custom instruction system as requested in #6098:

1. Support for External Instruction Files with Path Reference

• Users can now reference external instruction files by specifying their relative or absolute path
• Example: ./.github/copilot-instructions.md
• The system will attempt to load the file content; if the file doesn't exist, it falls back to treating the path as a literal instruction string
• This allows reusing instruction templates across multiple projects

2. Extension-Based Instruction Targeting

• Introduced a new function loadExtensionSpecificInstructions that loads instructions based on file extensions
• Supports naming convention like py-instruction.md for Python files, ts-instruction.md for TypeScript files, etc.
• The system checks for these files in:
  • Global .roo directory
  • Project-local .roo directory
  • Project root directory (for legacy support)
• This enables precise, language-aware context based on the file being edited

Changes

• Modified addCustomInstructions to support loading instructions from file paths
• Added new loadExtensionSpecificInstructions function for extension-based targeting
• Added comprehensive tests for both features
• Exported the new function for future integration

Testing

• Added 14 new test cases covering various scenarios
• All existing tests continue to pass
• Tests cover edge cases like non-existent files, directories, absolute/relative paths, and multiple instruction sources

Fixes #6098

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Important

This PR adds support for external instruction files and extension-based targeting in RooCode's custom instruction system, enhancing flexibility and scalability.

• Behavior:
  • addCustomInstructions now supports loading instructions from file paths, falling back to literal strings if files don't exist.
  • New loadExtensionSpecificInstructions function loads instructions based on file extensions, checking .roo directories and project root.
• Functions:
  • Added loadInstructionFromPath to read instructions from file paths in custom-instructions.ts.
  • Modified addCustomInstructions to integrate file path support.
  • Added loadExtensionSpecificInstructions to handle extension-based targeting.
• Testing:
  • Added 14 new test cases in custom-instructions.spec.ts for various scenarios including non-existent files and multiple instruction sources.
  • Tests cover edge cases like directories, absolute/relative paths, and extension-specific instructions.

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[daniel-lxs]

Closing, see #6098 (comment)