fix(coding-agent): preserved bash status while syncing cwd

Propagated the native shell working directory in ShellRunResult so AgentSession can refresh cwd without running a hidden pwd command in the persistent shell.

Added regression coverage for cd plus a failing command followed by echo $?, proving cwd sync no longer overwrites the user's last shell status.

Fixes #3958

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

@@ -51,6 +51,7 @@ export interface BashResult {
 	outputLines: number;
 	outputBytes: number;
 	artifactId?: string;
+	workingDir?: string;
 }
 
 const shellSessions = new Map<string, Shell>();
@@ -429,6 +430,7 @@ export async function executeBash(command: string, options?: BashExecutorOptions
 		return {
 			exitCode: winner.result.exitCode,
 			cancelled: false,
+			workingDir: winner.result.workingDir,
 			...(await sink.dump()),
 		};
 	} catch (err) {