fix(coding-agent): aligned output counters after sink replacement

- Adjusted OutputSink to disable head retention after replace(), resetting counters so later pushes append to the tail and do not trigger stale middle-elision in dump().
- Refined artifact link emission to insert a newline separator only when the minimized output lacked one.
- Added a regression test for replace-plus-push ordering that verifies no elision marker and aligned byte counts.

packages/coding-agent/src/exec/bash-executor.ts

@@ -252,7 +252,8 @@ export async function executeBash(command: string, options?: BashExecutorOptions
 					outputBytes: minimized.outputBytes,
 				});
 				if (artifactId) {
-					sink.push(`\n[raw output: artifact://${artifactId}]\n`);
+					const sep = minimized.text.endsWith("\n") ? "" : "\n";
+					sink.push(`${sep}[raw output: artifact://${artifactId}]\n`);
 				}
 			}
 		}