Fix streaming handling, add CLI mode, and update docs

[exemplarov]

Summary

• Add cli subcommand for one-shot requests (prompt/file input, optional output, default model gpt-5.3-codex, stdout streaming with trailing newline).
• Keep serve as default server mode and document it.
• Fix streaming behavior to avoid duplicate content.
• Route streaming responses through the real backend instead of a hardcoded placeholder.

Changes

• Added CLI plumbing and streaming output capture.
• Refactored backend request creation into a shared helper.
• Added real SSE parsing for CLI streaming.
• Updated README with CLI usage and serve examples.
• Removed old placeholder streaming response module.