🤖 Fix niceness bash escaping regression (#167)

Fixes the regression introduced in PR #162 where complex multi-line bash
scripts with conditionals would fail with exit code 2.

## Problem

The original implementation wrapped commands using `JSON.stringify`,
which converted newlines to literal `\n` strings:
```bash
nice -n 19 bash -c "if [ \0 -ne 0 ]; then\n  echo \"test\"\nfi"
```

This broke bash syntax, causing scripts with conditionals and multi-line
logic to fail.

## Solution

Spawn `nice` directly with proper arguments array instead of string
wrapping:
```typescript
spawn("nice", ["-n", "19", "bash", "-c", script], ...)
```

This avoids all escaping issues while maintaining the same niceness
functionality.

## Testing

Added comprehensive test coverage:
- ✅ Complex multi-line scripts with conditionals (reproduces the git
status regression)
- ✅ Exit code handling with niceness
- ✅ Simple commands with niceness

All 36 tests pass including the new niceness test suite.

_Generated with `cmux`_

Author: ammar-agent

src/services/tools/bash.test.ts

@@ -677,3 +677,88 @@ describe("bash tool", () => {
     }
   });
 });
+
+describe("niceness parameter", () => {
+  it("should execute complex multi-line scripts with niceness", async () => {
+    const tool = createBashTool({ cwd: process.cwd(), niceness: 19 });
+
+    // Complex script with conditionals, similar to GIT_STATUS_SCRIPT
+    const args: BashToolArgs = {
+      script: `
+# Test complex script with conditionals
+VALUE=$(echo "test")
+
+if [ -z "$VALUE" ]; then
+  echo "ERROR: Value is empty"
+  exit 1
+fi
+
+# Another conditional check
+RESULT=$(echo "success")
+if [ $? -ne 0 ]; then
+  echo "ERROR: Command failed"
+  exit 2
+fi
+
+echo "---OUTPUT---"
+echo "$VALUE"
+echo "$RESULT"
+`,
+      timeout_secs: 5,
+      max_lines: 100,
+    };
+
+    const result = (await tool.execute!(args, mockToolCallOptions)) as BashToolResult;
+
+    expect(result.success).toBe(true);
+    if (result.success) {
+      expect(result.output).toContain("---OUTPUT---");
+      expect(result.output).toContain("test");
+      expect(result.output).toContain("success");
+      expect(result.exitCode).toBe(0);
+    }
+  });
+
+  it("should handle exit codes correctly with niceness", async () => {
+    const tool = createBashTool({ cwd: process.cwd(), niceness: 19 });
+
+    // Script that should exit with code 2
+    const args: BashToolArgs = {
+      script: `
+RESULT=$(false)
+if [ $? -ne 0 ]; then
+  echo "Command failed as expected"
+  exit 2
+fi
+`,
+      timeout_secs: 5,
+      max_lines: 100,
+    };
+
+    const result = (await tool.execute!(args, mockToolCallOptions)) as BashToolResult;
+
+    expect(result.success).toBe(false);
+    expect(result.exitCode).toBe(2);
+    // Error message includes stderr output
+    if (!result.success) {
+      expect(result.error).toMatch(/Command failed as expected|Command exited with code 2/);
+    }
+  });
+
+  it("should execute simple commands with niceness", async () => {
+    const tool = createBashTool({ cwd: process.cwd(), niceness: 10 });
+    const args: BashToolArgs = {
+      script: "echo hello",
+      timeout_secs: 5,
+      max_lines: 100,
+    };
+
+    const result = (await tool.execute!(args, mockToolCallOptions)) as BashToolResult;
+
+    expect(result.success).toBe(true);
+    if (result.success) {
+      expect(result.output).toBe("hello");
+      expect(result.exitCode).toBe(0);
+    }
+  });
+});

src/services/tools/bash.ts

@@ -94,16 +94,15 @@ export const createBashTool: ToolFactory = (config: ToolConfiguration) => {
       }
 
       // Create the process with `using` for automatic cleanup
-      // If niceness is specified, wrap the command with nice
-      const finalScript =
+      // If niceness is specified, spawn nice directly to avoid escaping issues
+      const spawnCommand = config.niceness !== undefined ? "nice" : "bash";
+      const spawnArgs =
         config.niceness !== undefined
-          ? `nice -n ${config.niceness} bash -c ${JSON.stringify(script)}`
-          : script;
-      const finalCommand =
-        config.niceness !== undefined ? ["bash", "-c", finalScript] : ["bash", "-c", script];
+          ? ["-n", config.niceness.toString(), "bash", "-c", script]
+          : ["-c", script];
 
       using childProcess = new DisposableProcess(
-        spawn(finalCommand[0], finalCommand.slice(1), {
+        spawn(spawnCommand, spawnArgs, {
           cwd: config.cwd,
           env: {
             ...process.env,