feat(vibe): migrate to SkillsIntegration from the old prompts-based MarkdownIntegration (#2336)

* feat(vibe): migrate to SkillsIntegration and inject user-invocable frontmatter
Switches VibeIntegration from the old prompts-based MarkdownIntegration to SkillsIntegration, adopting the .vibe/skills/speckit-<name>/SKILL.md layout required by Mistral Vibe v2.0.0+. Post-processes each generated SKILL.md to inject `user-invocable: true` so skills are directly callable by users, not  just by other agents.

* test(vibe): assert user- invocable: true is present in all generated SKILL.md files

* Update tests/integrations/test_integration_vibe.py

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>