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/mod.rs

@@ -22,4 +22,4 @@ pub enum Commands {
     Storage(storage::StorageCommands),
     #[command(flatten)]
     Tools(tools::ToolsCommands),
-}
+}