feat: Universal cross-tool skill installation (14 platforms, format adapters, .agents/skills/)
- Fix install-template.sh POSIX compliance (set -eu, no bashisms, dash-tested)
- Fix incorrect paths: Windsurf → .windsurf/rules/, Codex → ~/.agents/skills/
- Add 7 new platforms: universal, kiro, trae, goose, opencode, roo-code, antigravity
- Add format adapters: auto-generate .mdc (Cursor), .md rules (Windsurf), plain .md (Cline/Roo/Trae)
- Add ~/.agents/skills/ universal secondary symlink after every install
- Add --all flag to install to every detected tool at once
- Add 3-tier platform support in cross-platform-guide.md (Native/Adapter/Manual)
- Update all 6 files with consistent paths across 14 platforms
- Fix set -e safety: replace ||/&& chaining with if/then/fi in detect_all_platforms
- Fix awk idempotency: exact marker match prevents substring collision
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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