Add langchain-pop integration (Persona Object Protocol middleware)

[Bin Zhang (joy7758)]

Summary

This PR adds documentation for langchain-pop, a middleware-style toolkit that enables Persona Object Protocol (POP) integration with LangChain agents.

The package is available on PyPI:

pip install langchain-pop

Repository:

• https://github.com/joy7758/langchain-pop
• https://pypi.org/project/langchain-pop/

What this adds

• A new middleware integration page for langchain-pop
• An entry in the Python middleware integrations index
• Redirects for underscore-to-hyphen route variants

Package capabilities

• POP v1 canonical object loading
• Automatic legacy pop-0.1 migration
• Persona-aware system prompt construction
• Boundary-aware tool filtering
• create_pop_agent(...) integration for LangChain agents

Validation

I tested the code examples against the published package before opening this PR:

• create_pop_agent(...) returns CompiledStateGraph
• POPMiddleware.filter_tools(...) filters blocked tools as documented
• Legacy pop-0.1 input migration works with the documented example shape

Notes for reviewers

• I placed this under python/integrations/middleware/ rather than tools/ because the integration is centered on LangChain middleware behavior and runtime tool filtering.
• The docs page avoids nonexistent APIs and uses the published langchain-pop package interface.

AI disclosure

This PR was prepared with assistance from an AI coding agent, then reviewed and validated before submission.

[Bin Zhang (joy7758)]

For reference, the POP prototype now also includes a small first-trial entry with a dry-run team demo.

• Try POP in 3 Minutes
• 3 portable persona objects projected into 3 CrewAI-style runtime roles

Repo:
https://github.com/joy7758/persona-object-protocol

[Bin Zhang (joy7758)]

One framing point I'm converging on: POP is a portable persona object layer for agent runtimes. Runtime configurations are derived projections, not the protocol core.