feat: add repoUrl field to skills for GitHub/repository links

[ScotTFO]

Summary

Adds a repoUrl field to the skills table so published skills can link back to their GitHub repo (or any repository).

Changes

Schema (convex/schema.ts)

• Added repoUrl: v.optional(v.string()) to the skills table

Backend (convex/skills.ts)

• On publish (create): extracts homepage, repository, or repo from YAML frontmatter → saves as repoUrl
• On publish (update): same extraction, falls back to existing repoUrl if frontmatter doesn't include one
• Priority order: homepage > repository > repo

Frontend (src/components/SkillDetailPage.tsx)

• Displays a clickable link in the skill hero section when repoUrl is present
• GitHub-aware label: shows "🔗 GitHub" for github.com URLs, "🔗 Repository" otherwise
• Opens in new tab with noopener noreferrer

How it works

Skill authors already include homepage in their SKILL.md YAML frontmatter:

---
name: my-skill
description: Does cool stuff
homepage: https://github.com/user/my-skill
---

This PR reads that field during publish and persists it on the skill record, then displays it on the detail page. Existing skills will get their repoUrl populated on their next publish.

No migration needed

The field is v.optional(v.string()), so existing skills simply show no link until they republish.

[vercel]

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