Add Buddy plugin SDK and unified tool registry

[owenqwenstarsky]

Summary

• add a public @teichai/buddy/plugin SDK for defining plugins, tools, and conditional approval requests
• auto-load compiled ESM plugins from ~/.buddy/plugins and merge plugin tools with built-in tools through a shared registry/runtime
• surface plugin metadata in approvals and tool activity, update docs and packaging, and add coverage for plugin loading, registry behavior, and approval flows

Testing

• npm run check
• npm test
• npm run build
• built-output smoke test for plugin discovery and execution