chore: remove stale dotdir cavecrew mirrors

.junie/, .kiro/, .roo/, .agents/ held pre-cleanup cavecrew/SKILL.md
mirrors (plus an unused .agents/plugins/marketplace.json). Nothing in
the current install path reads them — bin/install.js handles Codex via
npx skills add, and Claude Code marketplace discovery uses
.claude-plugin/marketplace.json. CLAUDE.md marks these remove-on-sight.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: JuliusBrussee