feat(zeromq): Allow external zmq.asyncio.Context injection

[achimnol]

Summary

• Add support for injecting an external zmq.asyncio.Context through transport_opts["zctx"]
• When an external context is provided, it will not be destroyed when the transport is closed
• Add CLAUDE.md for development environment guide

Motivation

Applications using callosum may want to share a single ZMQ context across multiple Peer instances throughout the application lifecycle. This reduces resource overhead and follows ZMQ best practices of using one context per application.

Changes

• ZeroMQBaseTransport.__init__(): Check for transport_opts["zctx"] and use it if provided
• ZeroMQBaseTransport.close(): Skip context.destroy() for externally injected contexts
• Added tests for both external and internal context lifecycle management

Usage

import zmq.asyncio

# Application-wide context
app_zctx = zmq.asyncio.Context()

peer = Peer(
    bind=ZeroMQAddress("tcp://127.0.0.1:5020"),
    transport=ZeroMQRPCTransport,
    transport_opts={"zctx": app_zctx},
    ...
)

# Application is responsible for cleanup
app_zctx.destroy(linger=100)

Test plan

• test_external_context_injection: Verifies external context is used and not destroyed on close
• test_internal_context_destroyed_on_close: Verifies internal context is destroyed on close