Refactor HTTP response handling into focused modules

[ryanfowler]

Summary

• Split src/http/response.rs into focused stdout, stream, formatters, and metadata modules
• Add a shared streamed-response finalizer to remove duplicated clipboard, timing, and status handling
• Update AGENTS.md to describe the new response layout

Testing

• Added and reorganized unit tests around the new response modules
• Verified the change with the full Rust test suite and the serialized integration suite