fix: convert Windows paths to POSIX for Git Bash commands

Add toPosixPath() utility that uses cygpath to convert Windows paths
(C:\foo\bar) to POSIX format (/c/foo/bar) for Git Bash compatibility.

Apply to:
- LocalBaseRuntime.spawnBackground: exitCodePath, cwd, stdoutPath, stderrPath
- WorktreeRuntime.deleteWorkspace: deletedPath for rm -rf
- runtimeFactory.getDefaultBgOutputDir: refactored to use shared utility

Author: ethanndickson

src/node/runtime/LocalBaseRuntime.ts

@@ -34,6 +34,7 @@ import {
 import { LocalBackgroundHandle } from "./LocalBackgroundHandle";
 import { buildWrapperScript, buildSpawnCommand } from "./backgroundCommands";
 import { log } from "@/node/services/log";
+import { toPosixPath } from "@/node/utils/paths";
 
 /**
  * Abstract base class for local runtimes (both WorktreeRuntime and LocalRuntime).
@@ -359,17 +360,18 @@ export abstract class LocalBaseRuntime implements Runtime {
     await fsPromises.writeFile(stderrPath, "");
 
     // Build wrapper script and spawn command using shared builders (same as SSH for parity)
+    // On Windows, convert paths to POSIX format for Git Bash (C:\foo → /c/foo)
     const wrapperScript = buildWrapperScript({
-      exitCodePath,
-      cwd: options.cwd,
+      exitCodePath: toPosixPath(exitCodePath),
+      cwd: toPosixPath(options.cwd),
       env: { ...options.env, ...NON_INTERACTIVE_ENV_VARS },
       script,
     });
 
     const spawnCommand = buildSpawnCommand({
       wrapperScript,
-      stdoutPath,
-      stderrPath,
+      stdoutPath: toPosixPath(stdoutPath),
+      stderrPath: toPosixPath(stderrPath),
       bashPath: getBashPath(),
       niceness: options.niceness,
       useSetsid: process.platform !== "win32",

src/node/runtime/WorktreeRuntime.ts

@@ -17,6 +17,7 @@ import { getProjectName } from "@/node/utils/runtime/helpers";
 import { getErrorMessage } from "@/common/utils/errors";
 import { expandTilde } from "./tildeExpansion";
 import { LocalBaseRuntime } from "./LocalBaseRuntime";
+import { toPosixPath } from "@/node/utils/paths";
 
 /**
  * Worktree runtime implementation that executes commands and file operations
@@ -285,7 +286,8 @@ export class WorktreeRuntime extends LocalBaseRuntime {
           }
 
           // Force delete the directory (use bash shell for rm -rf on Windows)
-          using rmProc = execAsync(`rm -rf "${deletedPath}"`, { shell: getBashPath() });
+          // Convert to POSIX path for Git Bash compatibility on Windows
+          using rmProc = execAsync(`rm -rf "${toPosixPath(deletedPath)}"`, { shell: getBashPath() });
           await rmProc.result;
 
           return { success: true, deletedPath };

src/node/runtime/runtimeFactory.ts

@@ -1,4 +1,3 @@
-import { execSync } from "child_process";
 import * as os from "os";
 
 import type { Runtime } from "./Runtime";
@@ -8,6 +7,7 @@ import { SSHRuntime } from "./SSHRuntime";
 import type { RuntimeConfig } from "@/common/types/runtime";
 import { hasSrcBaseDir } from "@/common/types/runtime";
 import { isIncompatibleRuntimeConfig } from "@/common/utils/runtimeCompatibility";
+import { toPosixPath } from "@/node/utils/paths";
 
 // Re-export for backward compatibility with existing imports
 export { isIncompatibleRuntimeConfig };
@@ -19,17 +19,7 @@ export { isIncompatibleRuntimeConfig };
  */
 function getDefaultBgOutputDir(): string {
   const tempDir = os.tmpdir();
-  if (process.platform === "win32") {
-    try {
-      // cygpath converts Windows paths to POSIX format for Git Bash / MSYS2
-      const posixPath = execSync(`cygpath -u "${tempDir}"`, { encoding: "utf8" }).trim();
-      return `${posixPath}/mux-bashes`;
-    } catch {
-      // Fallback if cygpath unavailable (shouldn't happen with Git Bash)
-      return "/tmp/mux-bashes";
-    }
-  }
-  return `${tempDir}/mux-bashes`;
+  return `${toPosixPath(tempDir)}/mux-bashes`;
 }
 
 /**

src/node/utils/paths.ts

@@ -0,0 +1,19 @@
+import { execSync } from "child_process";
+
+/**
+ * Convert a path to POSIX format for Git Bash on Windows.
+ * On non-Windows platforms, returns the path unchanged.
+ *
+ * Use this when building shell command strings that will run in Git Bash,
+ * where Windows-style paths (C:\foo\bar) don't work.
+ */
+export function toPosixPath(windowsPath: string): string {
+  if (process.platform !== "win32") return windowsPath;
+  try {
+    // cygpath converts Windows paths to POSIX format for Git Bash / MSYS2
+    return execSync(`cygpath -u "${windowsPath}"`, { encoding: "utf8" }).trim();
+  } catch {
+    // Fallback if cygpath unavailable (shouldn't happen with Git Bash)
+    return windowsPath;
+  }
+}