feat(server): forward per-prompt actor receipts

[alexanderkreidich]

Summary

• accept an optional opaque actor receipt on session prompt requests
• associate receipts with the exact delivered user message, including queued steering prompts
• inject the active receipt into provider metadata after project extensions
• remove stale or request-forged receipt metadata when no trusted receipt is active

Why

Gateways that authenticate end users need a trusted way to carry per-prompt attribution through agent-server to an OpenAI-compatible model proxy. The receipt remains opaque to agent-server; verification stays at the gateway's model proxy boundary.

Safety

• receipts are not added to prompt text, transcripts, SSE events, or tool environments
• concurrent sessions retain independent attribution
• steering switches attribution only when Pi delivers the queued user message
• rejected prompts and disposed sessions clear queued state

Validation

• npm test --workspace @appx-org/agent-server
• npm run typecheck --workspace @appx-org/agent-server
• Biome checks for changed files
• git diff --check