harness: add Pi Coding Agent support

[willwashburn]

Summary

Add Pi Coding Agent as a Relay-supported coding harness.

Context

Pi exposes a terminal coding-agent surface and RPC mode. RPC may map better than raw PTY injection for Relay workflows and result collection.

Sources:

• https://pi.dev/docs/latest/quickstart
• https://pi.dev/docs/latest/rpc

Scope

• Evaluate CLI vs RPC integration modes.
• Add CLI metadata and command builder support if the CLI path is viable.
• If RPC mode is stronger, design a process/headless adapter instead of PTY-only support.
• Add output/result collection tests.
• Add docs and smoke examples.

Acceptance

• Pi has a documented Relay integration path or a clear no-go rationale.
• Supported mode can execute a coding task and return output/result data reliably.
• Docs include install, auth/provider setup, transport, and limitations.