feat: Support custom instruction file paths

[roomote]

Summary

This PR implements support for custom instruction file paths as requested in #6168. Users can now load instruction files from paths outside the default .roocode/ folder, enabling teams to reuse shared instruction templates across projects.

Changes

• Added customInstructionPaths field to global settings schema
• Implemented loadCustomInstructionFiles function to load instructions from external paths
• Added comprehensive security validation:
  • Files must be within the workspace or parent directories (for monorepo scenarios)
  • Only markdown (.md, .markdown) and text (.txt) files are allowed
  • File size limit of 1MB to prevent loading huge files
  • Path validation to prevent dangerous patterns
• Updated state management to pass customInstructionPaths through ClineProvider
• Updated system prompt generation to include custom instruction paths
• Added comprehensive tests covering all edge cases and security scenarios
• Added JSDoc documentation for the new functionality

Usage Example

Users can now configure custom instruction paths in their settings:

{
  "customInstructionPaths": [
    ".github/copilot-instructions.md",
    "../shared/ai-instructions.md",
    "docs/team-guidelines.md"
  ]
}

Security Considerations

• Files are validated to ensure they're within the workspace or parent directories
• Absolute paths outside the workspace hierarchy are rejected
• Only text-based files (markdown and txt) are supported
• Large files (>1MB) are automatically skipped

Testing

• Added comprehensive unit tests for all functionality
• All existing tests pass
• Security validation is thoroughly tested

Fixes #6168

[daniel-lxs]

Closing for now, the issue requires scoping around the UX part, usually the main configuration file is not accessible by users