Rough Idea: Add Comprehensive Pi Agent Support to Ralph Orchestrator

Add pi-coding-agent (pi) as a first-class backend in ralph-orchestrator, on par with Claude, Kiro, Gemini, Codex, Amp, Copilot, and OpenCode.

Pi is a TypeScript-based coding agent CLI (@mariozechner/pi-coding-agent) that supports:

• Print mode (pi -p "prompt") — headless execution, outputs text or JSON
• JSON streaming (pi -p --mode json "prompt") — NDJSON event stream with pi-specific event types
• RPC mode (pi --mode rpc) — bidirectional JSON protocol over stdin/stdout for rich integration
• Interactive mode (pi "prompt") — TUI with initial prompt injection
• SDK (@mariozechner/pi-coding-agent npm package) — programmatic TypeScript API

Pi's NDJSON schema differs from Claude's stream-json: events are wrapped in pi-specific types (agent_start, message_update with assistantMessageEvent, tool_execution_start/end, etc.) rather than raw Anthropic Messages API events.

Key Integration Points

1. CliBackend: New pi() and pi_interactive() constructors
2. Auto-detection: Add pi to the detection priority list
3. Stream parsing: Handle pi's NDJSON format for real-time output in Ralph's TUI/console
4. RPC mode: Potentially leverage pi's RPC for richer features (steering, follow-ups, compaction)
5. Configuration: Support pi-specific config options (provider, model, thinking level, extensions, skills)
6. Cost tracking: Parse pi's usage/cost data from NDJSON events
7. Completion detection: Parse pi's agent_end events for LOOP_COMPLETE detection