Import plugins from repository marketplaces

[charlesgong-openai]

Summary

• discover Claude plugin marketplaces from supported repository paths during import
• process repository plugin migrations before home migrations and inherit matching home enabledPlugins intent
• prefer the repository marketplace source while keeping extraKnownMarketplaces as a fallback
• add warnings for missing marketplace sources, manifests, plugins, config reloads, and reinstall failures

Why

Repository marketplaces such as .claude-plugin/marketplace.json are not necessarily listed in extraKnownMarketplaces. As a result, an enabled plugin such as build-cop@treehouse could be visible to repository plugin discovery but could not be reinstalled by /import.

Root cause

The plugin catalog and external-agent import paths used different marketplace inputs. Catalog listing received repository working directories, while import resolved only settings-declared marketplaces and did not reconcile home enabledPlugins with the repository marketplace.

Impact

• /import can reinstall an enabled plugin directly from its repository marketplace
• repository marketplace discovery reuses the existing marketplace manifest loader
• duplicate repository/home plugin candidates are collapsed with repository scope winning
• the normal Codex marketplace install/cache path remains unchanged

Scope

This PR contains only Claude/repository plugin marketplace migration. Cursor migration remains in the separate existing PR.

Validation

• focused repository-marketplace tests: 3 passed
• just fix -p codex-app-server — passed
• just fmt — passed
• just test -p codex-app-server — 928 passed, 13 failed, 6 timed out, 1 skipped; non-passing cases are environment-only sandbox, missing test-binary, file-watcher, and image fixture failures unrelated to this change