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

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 #2232

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Start by reading Validate-SkillStructure.ps1 and locate the checks that distinguish Python skill directories from other skills. Run the validator against a Python skill missing uv.lock and a non-Python skill, then confirm the former produces a clear uv lock warning without failing and the latter behaves unchanged.

Written by the indexing model from the issue text.