docs(plugins): add comprehensive plugin development documentation

Add documentation in /docs/plugins/:
- README.md: Main plugin development guide with architecture, manifest, and capabilities
- GETTING_STARTED.md: Step-by-step tutorial for creating first plugin
- HOOKS.md: Complete reference for all 35+ hook types
- SECURITY.md: Security model including WASM sandboxing, resource limits, and best practices

Author: echobt