Fix imagegen skill placement under .system (#348)

## Summary
- Move the `imagegen` skill from `skills/.curated` into `skills/.system`
- Preserve the skill contents and assets under the new installed
location
- Update the CLI reference to point at the new system path

## Testing
- Not run (not requested)

skills/.system/imagegen/references/cli.md

@@ -16,7 +16,7 @@ Set a stable path to the skill CLI (default `CODEX_HOME` is `~/.codex`):
 
 ```
 export CODEX_HOME="${CODEX_HOME:-$HOME/.codex}"
-export IMAGE_GEN="$CODEX_HOME/skills/imagegen/scripts/image_gen.py"
+export IMAGE_GEN="$CODEX_HOME/skills/.system/imagegen/scripts/image_gen.py"
 ```
 
 Install dependencies into that environment with its package manager. In uv-managed environments, `uv pip install ...` remains the preferred path.