Add guide section for per-command env and cwd options

Add comprehensive guide section showing how to use per-command environment
variables and working directory options. Updates troubleshooting to recommend
the cwd option as best practice.

Changes:
- New section on setting environment variables and working directory
- Examples showing env and cwd options individually and combined
- Clear explanation that values do not persist
- Updated troubleshooting to show cwd as best practice

Synced from cloudflare/sandbox-sdk PR #204

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: github-actions[bot]

src/content/docs/sandbox/guides/execute-commands.mdx

@@ -133,6 +133,42 @@ await sandbox.exec('python /workspace/analyze.py data.csv');
 ```
 </TypeScriptExample>
 
+## Set environment variables and working directory
+
+Use per-command options to temporarily set environment variables or change the working directory for a specific command without affecting the session state:
+
+<TypeScriptExample>
+```
+// Set environment variables for a single command
+const result = await sandbox.exec('python train.py', {
+  env: {
+    MODEL_PATH: '/workspace/models',
+    BATCH_SIZE: '32',
+    LEARNING_RATE: '0.001'
+  }
+});
+
+// Run command in specific directory
+await sandbox.exec('npm run build', {
+  cwd: '/workspace/my-project'
+});
+
+// Combine both options
+await sandbox.exec('node server.js', {
+  env: { NODE_ENV: 'production', PORT: '3000' },
+  cwd: '/workspace/backend'
+});
+
+// Environment variables are scoped to the command only
+const check = await sandbox.exec('echo $MODEL_PATH');
+console.log(check.stdout); // Empty - MODEL_PATH does not persist
+```
+</TypeScriptExample>
+
+:::note
+Per-command `env` and `cwd` options are temporary and do not persist to the session. For persistent environment variables and working directories, create a [session](/sandbox/api/sessions/) with `createSession()`.
+:::
+
 ## Best practices
 
 - **Check exit codes** - Always verify `result.success` and `result.exitCode`
@@ -158,14 +194,19 @@ if (!check.success) {
 
 ### Working directory issues
 
-Use absolute paths or change directory:
+Use the `cwd` option for reliable directory changes:
 
 <TypeScriptExample>
 ```
-// Use absolute path
+// Best practice - use cwd option
+await sandbox.exec('python script.py', {
+  cwd: '/workspace/my-app'
+});
+
+// Alternative - use absolute path
 await sandbox.exec('python /workspace/my-app/script.py');
 
-// Or change directory
+// Works but less reliable - change directory in command
 await sandbox.exec('cd /workspace/my-app && python script.py');
 ```
 </TypeScriptExample>