Fix sk CLI compatibility with sharded registry artifacts

[majiayu000]

Summary

Closes #1.

• Follow registry compatibility pointers to search manifests, search shards, category manifests, and category parts.
• Resolve short-name installs from the current search index instead of the deprecated full registry.json payload.
• Normalize standard SKILL.md install refs so they install the containing skill directory, while filtering command markdown refs from search/category output.
• Deduplicate repeated registry entries by install command and prefer the shorter skill display name.
• Add focused tests for manifest following, category parts, dedupe/filtering, and GitHub path parsing.

Validation

• go test ./...
• go build -o /tmp/sk-test . && /tmp/sk-test search testing
• /tmp/sk-test search --category testing
• HOME=$(mktemp -d) /tmp/sk-test install docx
• HOME=$(mktemp -d) /tmp/sk-test install anthropics/skills/skills/docx/SKILL.md --name docx-file-path