Clarify manual setup for HITL invocations sample

[Jiahui-Gu]

Summary

• document the required Foundry project and model environment values for Bash and PowerShell
• add an authenticated-shell fallback when local azd credential discovery cannot invoke Azure CLI
• link the manual run back to the representative approval-flow invocation

Verification

• started the agent locally from the repaired branch
• verified readiness on the assigned loopback port
• submitted a proposal task, polled the awaiting-approval state, and approved it to completion