C-134: E2E: agent-to-agent settlement via MCP (autonomous)

**Owner:** @mehmethayirli  ·  **Reviewer:** @kh0ra
**Milestone:** M10 QA, E2E & final tests

Two keypair'd agents complete a full paid loop unattended on devnet.

**Acceptance criteria**
- Autonomous agent-to-agent loop green.

_Tracked in docs/MAINNET_ROADMAP.md (C-134)._