Fix code quality: trailing whitespace, unused imports, clippy warnings

- Fix trailing whitespace in kex.rs and s3.rs
- Add missing KexProposal import in kex_complete.rs
- Auto-fix clippy warnings across all crates
- All 153 tests pass

markbase-core/src/cli/tools/render.rs

@@ -23,4 +23,4 @@ pub fn handle_render_command(cmd: RenderCommand) -> anyhow::Result<()> {
         }
     }
     Ok(())
-}
+}