feat(core): implement SandboxManager interface and config schema

- Add `sandbox` block to `ConfigSchema` with `enabled`, `allowedPaths`,
  and `networkAccess` properties.
- Define the `SandboxManager` interface and request/response types.
- Implement `NoopSandboxManager` fallback that silently passes commands
  through but rigorously enforces environment variable sanitization via
  `sanitizeEnvironment`.
- Update config and sandbox tests to use the new `SandboxConfig` schema.
- Add `createMockSandboxConfig` utility to `test-utils` for cleaner test
  mocking across the monorepo.

packages/cli/src/utils/sandbox.ts

@@ -217,6 +217,7 @@ export async function start_sandbox(
 
     // runsc uses docker with --runtime=runsc
     const command = config.command === 'runsc' ? 'docker' : config.command;
+    if (!command) throw new FatalSandboxError('Sandbox command is required');
 
     debugLogger.log(`hopping into sandbox (command: ${command}) ...`);
 
@@ -230,6 +231,7 @@ export async function start_sandbox(
     const isCustomProjectSandbox = fs.existsSync(projectSandboxDockerfile);
 
     const image = config.image;
+    if (!image) throw new FatalSandboxError('Sandbox image is required');
     const workdir = path.resolve(process.cwd());
     const containerWorkdir = getContainerPath(workdir);