Parent Agent foundation

Droid gets its first serious Parent Agent foundation: a native shell, supervision loop, vendored runtime, build script, and orchestration path.

This is also where Pi joins the provider lineup, which is a pretty big deal for a 0.0.x release wearing a fake mustache.

• Added Parent Agent shell and supervision
• Vendored and bundled the Parent Agent runtime
• Added a build script for the Parent Agent runtime
• Refactored the coding-agent registry
• Added Pi as a coding agent provider
• Added DroidKit scripts and improved command-list rendering
• Improved Parent Agent attachment handling