When installing from a custom GitHub URL, the installer now analyzes
marketplace.json plugin structures to determine how to locate module
registration files (module.yaml, module-help.csv). Five strategies
are tried in cascade:
1. Root module files at the common parent of listed skills
2. A -setup skill with registration files in its assets/
3. Single standalone skill with registration files in assets/
4. Multiple standalone skills, each with their own registration files
5. Fallback: synthesize registration from marketplace.json metadata
and SKILL.md frontmatter
Also changes the custom URL flow from confirm-all to multiselect,
letting users pick which plugins to install. Already-installed modules
are pre-checked for update; new modules are unchecked for opt-in.
New file: tools/installer/modules/plugin-resolver.js
Modified: custom-module-manager.js, official-modules.js, ui.js
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