* refactor(installer): remove custom content installation feature
Remove the entire local filesystem custom content feature from the
installer to make way for marketplace-based plugin installation.
Deleted: custom-handler.js, custom-module-cache.js, custom-modules.js
Removed: --custom-content CLI flag, interactive custom content prompts,
custom module caching, manifest tracking, missing-source resolution,
and related test suites. Updated docs across all translations.
* fix: address review findings from Augment
Fix admonition syntax (remove accidental space in :::note) across 4
translated docs files, and update stale JSDoc on listAvailable().
* feat(installer): fetch module list from marketplace registry
Switch module list source of truth from bundled
external-official-modules.yaml to the remote marketplace registry
(registry/official.yaml) fetched via raw.githubusercontent.com.
- Rewrite ExternalModuleManager to fetch from GitHub with local fallback
- Simplify selectAllModules/getDefaultModules to use registry as single source
- Registry order controls display order; built_in flag prevents cloning
- Rename fallback file to registry-fallback.yaml in modules/
- Only show legacy migration message when legacy dirs actually exist
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