Document `PlatformAuthBackend` and agent-to-agent authentication

[jenna-winkler]

Is your feature request related to a problem? Please describe.
PlatformAuthBackend is undocumented. Developers encounter it in sample agents but have no explanation of what it does, how it works, or when they should use it.

Describe the solution you'd like
Add documentation explaining the agent-to-agent authentication model in Agent Stack, including how context tokens and audience validation work, why agents should call each other through the agentstack-server gateway rather than directly, and where PlatformAuthBackend fits into that model.

Additional context

• Implementation: apps/agentstack-sdk-py/src/agentstack_sdk/server/middleware/platform_auth_backend.py
• Example usage: agents/chat/src/chat/agent.py#L292

[dosubot]

Hi, @jenna-winkler. I'm Dosu, and I'm helping the agentstack team manage their backlog and am marking this issue as stale.

Recap

• You raised a documentation gap around PlatformAuthBackend, which is currently undocumented and confusing in sample agents.
• The main request is for a clear explanation of Agent Stack’s agent-to-agent auth flow, including context tokens, audience validation, and why requests should go through the agentstack-server gateway.
• You also asked where PlatformAuthBackend fits into that request path.
• There haven’t been any follow-up comments or updates, so the issue remains centered on the original clarification request.

Next Steps

• If this is still relevant on the latest version of the agentstack repository, please comment here to keep the discussion open.
• Otherwise, this issue will be automatically closed in 7 days.

Thanks for your understanding and contribution.