feat(sdk): generate Python client from proto

[EngineerProjects]

Summary

Generate a Python gRPC client from nexus.proto and publish it as a
pip-installable package so Python developers can use Nexus without
running Go.

Deliverables

• sdk/python/ — generated stubs + thin wrapper with a friendly API
• sdk/python/nexus/__init__.py — NexusClient class
• sdk/python/pyproject.toml — installable as pip install nexus-engine
• README in sdk/python/README.md with quickstart

NexusClient API (proposed)

from nexus import NexusClient

client = NexusClient(host="localhost:50051", api_key="...")

# One-shot query
response = client.query("Refactor the auth module")
print(response.content)

# Streaming
for event in client.stream("Write a Go HTTP handler"):
    if event.HasField("text_chunk"):
        print(event.text_chunk.text, end="", flush=True)

# Sessions
session = client.create_session()
session.submit("First message")
session.submit("Follow-up question")

Acceptance criteria

• Generated stubs checked into sdk/python/nexus/_generated/
• NexusClient wraps stubs with a Pythonic API (no raw proto objects exposed)
• Works with Python 3.10+
• make sdk-python regenerates stubs from proto
• At least one working example in sdk/python/examples/
• Depends on: feat(grpc): improve proto — sessions, multi-agent RPCs, structured events #17 (improved proto with sessions)