fix: preserve structured ACP prompt blocks

[vincentkoc]

Summary

• add shared prompt-block parsing so stdin/file input can carry structured ACP content, including images
• thread structured prompt blocks through the runtime/client path while keeping queue-owner IPC backward-compatible via the legacy message summary
• add CLI and queue parsing coverage for structured prompt blocks in exec and queued prompt flows

Testing

• pnpm run build:test && node --test dist-test/test/cli.test.js dist-test/test/queue-messages.test.js