Refactor SSE, NDJSON, and gRPC streaming onto a shared stdout driver

[ryanfowler]

Summary

• Added a small formatter callback driver in src/http/mod.rs to centralize decoded reads, clipboard capture, byte counting, trailer extraction, and stdout flushing.
• Kept SSE, NDJSON, and gRPC parsing logic separate behind formatter-specific callbacks and local state objects.
• Reused the same trailer snapshot helper across the other streaming paths.
• Updated AGENTS.md to document the new streaming ownership boundary.

Testing

• cargo fmt
• cargo clippy --locked --all-targets --all-features -- -D warnings
• Focused Rust tests for formatted streaming paths passed, including SSE, NDJSON, gRPC, and split-chunk NDJSON coverage.