chore: Add soft uv.lock warning to Validate-SkillStructure.ps1

[github-actions]

Add a non-blocking warning to Validate-SkillStructure.ps1 when a Python skill directory is missing a uv.lock file, alerting contributors early without breaking the validation run.

Parent tracking issue: #2232

Acceptance Criteria

• Validate-SkillStructure.ps1 emits a warning (not an error) when a Python skill is missing a uv.lock file
• Warning message is clear and actionable (e.g., references uv lock command)
• Existing validation behavior for non-Python skills is unchanged
• Validation does not fail solely due to a missing uv.lock
  Related to chore: Dependabot uv coverage + dependency vulnerability remediation #2232

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