Fix duplicate skill discovery for mirrored SKILL.md files

[caioribeiroclw-pixel]

Summary

• de-duplicate repository-discovered skills by skill name before install/selection
• prefer canonical top-level skills/<name>/SKILL.md over example/docs/.well-known mirrors
• show which duplicate source paths were ignored so install provenance is explicit

Why

This addresses #92: repos can contain the same skill in canonical folders plus examples or discovery mirrors. The current installer reports every mirror as a separate skill and overwrites the same target directory repeatedly, which makes the install log ambiguous.

Checks

• npm run typecheck
• npm test
• git diff --check
• smoke: node dist/cli.js install caioribeiroclw-pixel/pluribus --universal --yes now reports 2 skills, warns about ignored mirrors, and writes metadata with subpath: skills/context-receipts