feat: skills: loader (part 3)

[kalenkevich]

Please ensure you have read the contribution guide before creating a pull request.

Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Related: Support for skills in the js adk #155

2. Or, if no issue exists, describe the change:

Problem:
The ADK needs a way to load and validate "Skills" defined as Markdown files with YAML frontmatter from local directories. This is part of the larger Skills feature.

Solution:
Implemented a skills file loader in core/src/skills/loader.ts. This module provides functionality to:

• Parse SKILL.md files, separating YAML frontmatter from the Markdown body.
• Load a full skill directory, including associated resources (assets, references, scripts) from subdirectories.
• Validate a skill directory structure and frontmatter without fully loading all assets.
• List all valid skills within a base directory.

Added comprehensive unit tests in core/test/skills/loader_test.ts to verify these behaviors.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Manual End-to-End (E2E) Tests:
N/A for this component-level feature. E2E tests will be covered in future PRs integrating skills with the runner.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end