VS Code Copilot Agent Skills do not support the `mode:` frontmatter field.
The generated SKILL.md files included `mode: speckit.<stem>` injected by
CopilotIntegration.post_process_skill_content(), which had no effect in
VS Code and could cause confusion. Simplify post_process_skill_content to
delegate directly to _CopilotSkillsHelper without injecting mode:.
Update tests to assert mode: is absent from generated skill frontmatter.
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>