fix: use bash shell for POSIX commands on Windows

Add shell option to execAsync so callers can specify bash for POSIX
commands that don't work with cmd.exe (Windows default).

Fixed:
- LocalBaseRuntime.spawnBackground: nohup/setsid commands
- WorktreeRuntime.deleteWorkspace: rm -rf command

Author: ethanndickson

src/node/runtime/LocalBaseRuntime.ts

@@ -376,7 +376,8 @@ export abstract class LocalBaseRuntime implements Runtime {
     });
 
     try {
-      using proc = execAsync(spawnCommand);
+      // Use bash shell explicitly - spawnCommand uses POSIX commands (nohup, setsid)
+      using proc = execAsync(spawnCommand, { shell: getBashPath() });
       const result = await proc.result;
 
       const pid = parseInt(result.stdout.trim(), 10);

src/node/runtime/WorktreeRuntime.ts

@@ -12,6 +12,7 @@ import type {
 import { listLocalBranches } from "@/node/git";
 import { checkInitHookExists } from "./initHook";
 import { execAsync } from "@/node/utils/disposableExec";
+import { getBashPath } from "@/node/utils/main/bashPath";
 import { getProjectName } from "@/node/utils/runtime/helpers";
 import { getErrorMessage } from "@/common/utils/errors";
 import { expandTilde } from "./tildeExpansion";
@@ -283,8 +284,8 @@ export class WorktreeRuntime extends LocalBaseRuntime {
             // Ignore prune errors - we'll still try rm -rf
           }
 
-          // Force delete the directory
-          using rmProc = execAsync(`rm -rf "${deletedPath}"`);
+          // Force delete the directory (use bash shell for rm -rf on Windows)
+          using rmProc = execAsync(`rm -rf "${deletedPath}"`, { shell: getBashPath() });
           await rmProc.result;
 
           return { success: true, deletedPath };

src/node/utils/disposableExec.ts

@@ -113,16 +113,27 @@ class DisposableExec implements Disposable {
   }
 }
 
+/**
+ * Options for execAsync.
+ */
+export interface ExecAsyncOptions {
+  /** Shell to use for command execution. If not specified, uses system default (cmd.exe on Windows). */
+  shell?: string;
+}
+
 /**
  * Execute command with automatic cleanup via `using` declaration.
  * Prevents zombie processes by ensuring child is reaped even on error.
  *
  * @example
  * using proc = execAsync("git status");
  * const { stdout } = await proc.result;
+ *
+ * // With explicit shell (needed for POSIX commands on Windows)
+ * using proc = execAsync("nohup bash -c ...", { shell: getBashPath() });
  */
-export function execAsync(command: string): DisposableExec {
-  const child = exec(command);
+export function execAsync(command: string, options?: ExecAsyncOptions): DisposableExec {
+  const child = exec(command, { shell: options?.shell });
   const promise = new Promise<{ stdout: string; stderr: string }>((resolve, reject) => {
     let stdout = "";
     let stderr = "";