Add documentation for synchronous workflows and signal APIs

## Summary

There is no dedicated design documentation for synchronous workflows and signal APIs. These are important features that need clear documentation covering API signatures, request parameters, request/response bodies, behaviors, examples, and best practices.

## Desired Outcome

A design document (e.g., `docs/design/SYNCHRONOUS_WORKFLOWS_AND_SIGNAL_APIS.md`) covering:

- Synchronous workflow execution API
- Signal API for sending signals to running workflows
- Request/response formats
- Behavioral details and edge cases
- Usage examples and best practices

## Prior Art

Community PR #666 by @v1r3n contained a comprehensive 547-line design document.

Start by reading community PR #666, which contains the prior 547-line design document. Create docs/design/SYNCHRONOUS_WORKFLOWS_AND_SIGNAL_APIS.md covering synchronous execution, signals, request and response formats, behaviors, edge cases, examples, and best practices; done means each requested area is documented.

Written by the indexing model from the issue text.