docs: add plugin documentation, remove automated sandbox setup

- Add docs/claude-code-plugin.md for plugin installation and usage
- Add docs/docker-sandbox.md for manual sandbox setup instructions
- Remove docker/ folder (automated setup didn't work with sandbox init)
- Remove --sandboxed flag from Ralph scripts
- Remove run-sandboxed-ralph.sh launcher
- Update README to link to plugin documentation

Sandbox users should manually install plugin/MCP after starting sandbox.

Author: Dylan Storey

README.md

@@ -93,15 +93,17 @@ Add to your `claude_desktop_config.json`:
 claude mcp add --scope user --transport stdio metis -- metis mcp
 ```
 
-**Optional: Install the Flight Levels Skill**
+**Optional: Install the Flight Levels Plugin**
 
-The Metis skill teaches Claude *when* and *why* to use Metis tools (methodology, decomposition patterns, anti-patterns):
+The Metis plugin teaches Claude *when* and *why* to use Metis tools (methodology, decomposition patterns, anti-patterns). It also provides commands like `/metis-ralph` for autonomous task execution.
 
 ```bash
 /plugin marketplace add colliery-io/metis
 /plugin install metis@colliery-io-metis
 ```
 
+See [Claude Code Plugin Documentation](docs/claude-code-plugin.md) for full details including available commands and skills.
+
 #### For Cursor
 Add to your Cursor MCP configuration:
 ```json