Added a streams demo that runs the agent loop on a provider.

[moedash]

This PR adds streams_demo, a small agent loop that runs unchanged on any stream provider.

What changed?

• streams_demo/agent_loop.py holds the workflow and activity. The workflow reads inputs, decides, publishes, and runs an activity that appends receipts to the workflow's stream.
• streams_demo/run_demo.py drives three cases: the loop with the smallest cache the provider supports, a producer whose second attempt supersedes its first, and an outside reader following what the workflow published.
• streams_demo/provider_setup.py is the one file that names a provider. Here it's MemoryStreams.
• pyproject.toml adds the directory to pyright's include.

Part of AI-198 (epic AI-37).

Why?

It shows the interface's main promise in a form you can run: the same workflow, activity and client code on any store. Each provider PR only has to change provider_setup.py to show it works the same way. It's a sample, not library code, so it gets its own PR.

How did you test it?

Link to a test plan if any -

• Unit Tests
• Staging
• End to End Tests

poe lint is clean, pyright included. I ran python streams_demo/run_demo.py against a local Temporal server. All three cases finished, and the activity's receipts showed up on the workflow's stream.