Readability pass: remove verbose comments and clean up tests

- completion.rs: Remove redundant comments, clean up test output (println! -> let _)
- g3_status.rs: Condense doc comments, rename from_str() to parse()
- streaming.rs: Remove obvious doc comments that duplicate function names
- simple_output.rs, ui_writer_impl.rs: Update Status::parse() calls

All changes are behavior-preserving. 132 lines removed, code is more scannable.

Agent: carmack

crates/g3-cli/src/ui_writer_impl.rs

@@ -234,7 +234,7 @@ impl UiWriter for ConsoleUiWriter {
     fn print_g3_status(&self, message: &str, status: &str) {
         use crate::g3_status::Status;
         let _ = message; // unused now - progress already printed the message
-        crate::g3_status::G3Status::status(&Status::from_str(status));
+        crate::g3_status::G3Status::status(&Status::parse(status));
     }
 
     fn print_thin_result(&self, result: &g3_core::ThinResult) {