Fix conservative openai.yaml regeneration and validate agents config

[wsk-builds]

Summary

• make generate_openai_yaml.py update agents/openai.yaml conservatively instead of overwriting non-interface sections
• make quick_validate.py validate agents/openai.yaml syntax and core structure in addition to SKILL.md
• add targeted regression tests covering metadata preservation and invalid YAML detection

Problem

generate_openai_yaml.py currently rewrites the entire agents/openai.yaml file from scratch. Re-running it can silently drop existing fields such as dependencies, icons, and default_prompt, even though those are documented as supported structure in references/openai_yaml.md.

quick_validate.py also only validates SKILL.md frontmatter today. A skill can contain an invalid or structurally broken agents/openai.yaml and still pass validation.

Changes

Conservative regeneration

• load any existing agents/openai.yaml before regenerating
• reject regeneration when the existing file is invalid YAML or has a non-mapping top-level/interface shape
• update the required interface.display_name and interface.short_description fields
• apply any explicit interface overrides passed on the command line
• preserve existing non-interface sections, including dependencies
• preserve existing interface keys that were not explicitly overridden

Validation coverage

• add agents/openai.yaml validation to quick_validate.py
• fail on invalid YAML, empty files, non-mapping top-level documents, non-mapping interface/dependencies, and malformed dependencies.tools
• check that the core documented string fields under interface and dependencies.tools[] have string values

Regression tests

• add a focused unittest module for the skill-creator scripts
• cover preservation of existing dependencies and interface metadata during regeneration
• cover rejection of invalid existing agents/openai.yaml
• cover validator success on a valid agents/openai.yaml and failure on invalid YAML

Validation

• python3 skills/.system/skill-creator/scripts/skill_creator_scripts_test.py
• python3 skills/.system/skill-creator/scripts/quick_validate.py skills/.system/skill-creator
• python3 skills/.system/skill-creator/scripts/quick_validate.py skills/.system/openai-docs