docs: specify server-managed prompt cache state for Messages

[franciscojavierarceo]

Summary

• define private, non-authoritative server-managed cache state for the Messages API
• preserve the conversation-stateless public Messages contract while coordinating affinity, checkpoints, retention, branches, and provider cache artifacts
• reconcile the proposal with the existing Claude Code integration design
• define the ownership boundary: Agentic API derives trusted logical coordinates and lifecycle policy, llm-d owns routed worker selection, and vLLM owns physical KV execution
• link the reproducible affine KV benchmark evidence and qualify the uncommitted Claude Code capture
• provide staged rollout, failure semantics, security controls, observability, benchmarks, and acceptance criteria

Test Plan

• cargo test
• uv run --with-requirements docs/requirements.txt mkdocs build --strict
• git diff --check origin/main...HEAD
• verify linked vLLM, llm-d, affine ADR, and raw benchmark sources resolve