Implements a plugin system for ZeroClaw modeled after OpenClaw's architecture.
Key components:
- Plugin trait and PluginApi for registering tools/hooks
- Plugin manifest (zeroclaw.plugin.toml) for metadata
- Plugin discovery from bundled, global, and workspace directories
- PluginRegistry managing loaded plugins, tools, and hooks
- Error isolation via panic catching in register()
- Config integration via [plugins] section
Example plugin included in extensions/hello-world/.
Closes#1414
# Conflicts:
# src/config/mod.rs
# src/config/schema.rs
