feat: SubFlow, Visualization, and Pi Agent Example

[dahai9]

PR Description: SubFlow, Visualization, and Pi Agent

Summary

This PR introduces three major enhancements to the PocketFlow-Rust framework to improve modularity, usability, and developer experience.

1. SubFlow Node (Composition)

Introduced SubFlowNode<SubState, ParentState>, allowing a Flow to be nested within another Flow.

• Supports flexible context inheritance/isolation via context_builder.
• Supports dynamic state mapping via result_mapper.
• Enables building complex, recursive workflows from smaller, testable sub-flows.

2. Mermaid Visualization

Added to_mermaid() methods to Flow and BatchFlow.

• Automatically generates Mermaid TD flowchart strings representing the workflow graph.
• Simplifies debugging and documentation of complex agentic workflows.

3. Pi Agent Example

A high-performance Rust implementation of a coding agent using the PocketFlow-Rust framework.

• Modularized into a standalone workspace member (examples/pi).
• Features interactive CLI, tool-use (read/write/bash), and history compaction.
• Demonstrates real-world application of the framework's capabilities.

4. Simplified State Transitions

Extended ProcessState to automatically handle condition string generation via strum::Display.

• Reduces boilerplate and simplifies the build_flow! macro usage.

Technical Details

• No new core dependencies: Relies on existing serde, anyhow, and strum.
• Modularity: Moved agent-specific utilities to the example crate to keep the core library generic.
• Verification: Added unit tests for SubFlow and Mermaid visualization; verified pi agent functionality.

Verification

• cargo test (all unit tests pass)
• cargo check -p pocketflow_rs -p pi
• Manual execution of examples/pi and examples/basic.rs.

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Created with ❤️ by Antigravity