docs: add Pramagent policy gate to HITL sample

[sriram7737]

Adds an optional Pramagent policy-gate section to the async human-in-the-loop sample README.

Why:

• shows where to validate a proposed tool call before tool.run_json executes
• keeps the existing sample dependency-free and runnable as-is
• documents how AutoGen HITL can be combined with deterministic external tool policy

Validation:

• git diff --check

[sriram7737]

@microsoft-github-policy-service agree