fix: quote bash path and validate SSH cwd in spawnBackground

- Quote bash path in buildSpawnCommand to handle Windows paths with
  spaces (e.g., C:\Program Files\Git\bin\bash.exe)
- Add cwd validation to SSH spawnBackground (parity with local runtime)
- Add test for bash paths with spaces

Author: ethanndickson

src/node/runtime/SSHRuntime.ts

@@ -282,6 +282,15 @@ export class SSHRuntime implements Runtime {
   ): Promise<BackgroundSpawnResult> {
     log.debug(`SSHRuntime.spawnBackground: Spawning in ${options.cwd}`);
 
+    // Verify working directory exists on remote (parity with local runtime)
+    const cwdCheck = await execBuffered(this, cdCommandForSSH(options.cwd), {
+      cwd: "/",
+      timeout: 10,
+    });
+    if (cwdCheck.exitCode !== 0) {
+      return { success: false, error: `Working directory does not exist: ${options.cwd}` };
+    }
+
     // Generate unique process ID and compute output directory
     // /tmp is cleaned by OS, so no explicit cleanup needed
     const processId = `bg-${randomBytes(4).toString("hex")}`;

src/node/runtime/backgroundCommands.test.ts

@@ -110,7 +110,8 @@ describe("backgroundCommands", () => {
         stderrPath: "/tmp/stderr.log",
       });
 
-      expect(result).toMatch(/^\(setsid nohup bash -c /);
+      // bash path is quoted to handle paths with spaces (e.g., Windows Git Bash)
+      expect(result).toMatch(/^\(setsid nohup 'bash' -c /);
     });
 
     it("should include niceness prefix when provided", () => {
@@ -142,7 +143,20 @@ describe("backgroundCommands", () => {
         bashPath: "/usr/local/bin/bash",
       });
 
-      expect(result).toContain("/usr/local/bin/bash -c");
+      // bash path is quoted
+      expect(result).toContain("'/usr/local/bin/bash' -c");
+    });
+
+    it("should handle bash path with spaces (Windows Git Bash)", () => {
+      const result = buildSpawnCommand({
+        wrapperScript: "echo hello",
+        stdoutPath: "/tmp/stdout.log",
+        stderrPath: "/tmp/stderr.log",
+        bashPath: "/c/Program Files/Git/bin/bash.exe",
+      });
+
+      // Path with spaces must be quoted to work correctly
+      expect(result).toContain("'/c/Program Files/Git/bin/bash.exe' -c");
     });
 
     it("should redirect stdout and stderr", () => {

src/node/runtime/backgroundCommands.ts

@@ -103,7 +103,7 @@ export function buildSpawnCommand(options: SpawnCommandOptions): string {
   const quotePath = options.quotePath ?? shellQuote;
 
   return (
-    `(${nicePrefix}${setsidPrefix}nohup ${bash} -c ${shellQuote(options.wrapperScript)} ` +
+    `(${nicePrefix}${setsidPrefix}nohup ${shellQuote(bash)} -c ${shellQuote(options.wrapperScript)} ` +
     `> ${quotePath(options.stdoutPath)} ` +
     `2> ${quotePath(options.stderrPath)} ` +
     `< /dev/null & echo $!)`