feat(skills): browse + select nested GitHub-repo skills for a pack

[DIodide]

Importing mattpocock/skills (and any nested repo) failed with "no skills found." Fixes that, and upgrades repo import to the same power as npx skills add: browse, preview contents, and pick which skills to add.

The bug

mattpocock/skills nests skills as skills/<category>/<id>/SKILL.md, but discovery only matched a fixed set of top-level bases (skills, .claude/skills, .agents/skills) exactly — anything a folder deeper was invisible. (Content-fetching already resolved nested skills by leaf-dir name — both the Convex viewer and the gateway backfill — so only discovery was broken.)

Backend (convex/skills.ts)

• discoverRepoSkills finds every SKILL.md at any depth in one git/trees call, derives a display category by stripping a recognized skills-root prefix, and skips node_modules.
• New listRepoSkills action: discovers + fetches + caches each SKILL.md (so previews are instant) and returns them with category + description + the repo's AGENTS.md/CLAUDE.md — for a choose-what-you-want flow.
• importSkillRepo (curated templates' add-all) now shares the same discovery, so it gets nested support too. A repo that resolves but has no skills now says "no skills found," not "repo not found."

Frontend

• New RepoImportDialog — a two-pane browse-and-select modal:
  • Left: discovered skills grouped by category, with per-skill / per-group / select-all checkboxes and an "already added" state (pre-selects everything new).
  • Right: a live SKILL.md preview (rendered markdown, served from the browse cache → instant) so you can see exactly what a skill does before adding.
  • Footer: optionally also pull in AGENTS.md / CLAUDE.md.
• The pack editor's "Import a GitHub repo" now opens this dialog (Browse) instead of a blind "Import all."

Tests / checks

• convex/skills.test.ts stubs a nested (mattpocock-shaped) tree and pins discovery-at-depth, category derivation, node_modules exclusion, URL normalization, SKILL.md caching, and the no-skills-vs-repo-not-found messages.
• convex 239 passed, web production build succeeds, biome clean. (GITHUB_TOKEN is already set on the prod Convex deployment, so rate limits won't bite.)

After merge to staging I'll promote to main.