Feat: Add skill scaffolding and Flag cleanup skill

[nieblara]

This pull request sets up the initial public release of the LaunchDarkly Agent Skills repository. It introduces standards and automation for contributing, validating, and releasing agent skills, along with documentation and templates to support open-source collaboration.

Repository standards and documentation:

• Added README.md describing the repo purpose, available skills, installation instructions, and licensing.
• Added CONTRIBUTING.md with guidelines for skill authoring, naming, versioning, and contribution workflow.
• Added SECURITY.md with instructions for responsible disclosure of security issues.
• Added LICENSE specifying Apache-2.0 licensing for all contributions.
• Added CHANGELOG.md to track notable changes and releases.

Automation and validation workflows:

• Added GitHub Actions workflow for validating skills and catalog generation (.github/workflows/validate-skills.yml).
• Added Python scripts for validating skill metadata (scripts/validate_skills.py) and generating the skills catalog (scripts/generate_catalog.py). [1] [2]
• Added GitHub Actions workflow for packaging and releasing skills (.github/workflows/release.yml).

Templates for collaboration:

• Added issue templates for bug reports and feature requests, and a pull request template. [1] [2] [3]
• Added CODEOWNERS file for team ownership assignment.

Skills catalog and examples:

• Added initial skills.json catalog and example skill directories (skills/feature-flags/launchdarkly-flag-cleanup, skills/skill-authoring/create-skill) with supporting documentation. [1] [2]