Migrate runtime to Pi provider architecture

[VectorJet]

Summary

Migrate key runtime components to use the new Pi-based provider architecture (@earendil-works/pi-ai, @earendil-works/pi-agent-core) as described in skyth/pi/.

Tasks

• Install Pi packages and replace types.ts mirrors with real imports
• Implement PiProvider in provider.ts using the Pi streamSimple API
• Integrate PiProvider with StepRunner and verify end-to-end turns
• Move session routing/naming to Pi-backed completion helpers
• Remove legacy skyth/providers/* once migration is complete
• Update or add migration/integration tests for new Pi layer
• Update documentation and migration guides

References:

• skyth/pi/README.md
• specs/core/hybrid-agent-loop.md

Notes

• This migration is a core architectural update, improves future maintainability, and enables compatibility with next-gen agent patterns.
• Attempt to keep changes modular with migration behind a feature flag until stable.