Sync third-party skill plugins from upstream repos

Constraint: Maintainers need repeatable ingestion for external skill repositories with different layouts.\nRejected: Git submodules as the primary mechanism | Codex marketplace installs do not reliably recurse external submodules, and adapters still need layout conversion.\nConfidence: medium\nScope-risk: moderate\nDirective: Add new upstreams through config/external-sources.json and adapter logic rather than hand-copying plugin folders.\nTested: scripts/sync_external_plugins.ps1 -Proxy http://127.0.0.1:18085; scripts/sync_external_plugins.ps1 -Only ui-ux-pro-max -Proxy http://127.0.0.1:18085; python scripts/validate_marketplace.py; validate_plugin.py for superpowers and ui-ux-pro-max; codex plugin add superpowers@eapil-skill-market; codex plugin add ui-ux-pro-max@eapil-skill-market; verified ui-ux-pro-max data/scripts directories in cache.\nNot-tested: Scheduled .gitea workflow execution on git.playones runner.

README.md

@@ -57,6 +57,8 @@ Start a new Codex thread after installing so the new skills are loaded into cont
 - `eapil-threejs-postprocessing`
 - `eapil-threejs-shaders`
 - `eapil-threejs-textures`
+- `superpowers`
+- `ui-ux-pro-max`
 
 List available plugins:
 
@@ -70,6 +72,33 @@ Remove an installed plugin:
 codex plugin remove eapil-pdf@eapil-skill-market
 ```
 
+## External sync
+
+Third-party plugins are configured in `config/external-sources.json` and synced with:
+
+```powershell
+powershell -ExecutionPolicy Bypass -File .\scripts\sync_external_plugins.ps1
+```
+
+If GitHub access is unstable locally, use the proxy:
+
+```powershell
+powershell -ExecutionPolicy Bypass -File .\scripts\sync_external_plugins.ps1 -Proxy http://127.0.0.1:18085
+```
+
+The sync script currently supports:
+
+- `codex-plugin`: mirrors an upstream Codex plugin repo, used for `obra/superpowers`.
+- `claude-skill`: converts one Claude-style skill directory into a Codex plugin, used for `nextlevelbuilder/ui-ux-pro-max-skill`.
+
+After sync, validate the marketplace:
+
+```powershell
+python scripts\validate_marketplace.py
+```
+
+`.gitea/workflows/sync-external-skills.yml` is a scheduled workflow prototype for Git-side periodic sync.
+
 ## Layout
 
 ```text
@@ -85,4 +114,10 @@ plugins/codex/
   plugins/eapil-threejs-fundamentals/
     .codex-plugin/plugin.json
     skills/threejs-fundamentals/
+  plugins/superpowers/
+    .codex-plugin/plugin.json
+    skills/
+  plugins/ui-ux-pro-max/
+    .codex-plugin/plugin.json
+    skills/ui-ux-pro-max/
 ```