add support for -w,--writable-root to add more writable roots for sandbox (#263)

This adds support for a new flag, `-w,--writable-root`, that can be
specified multiple times to _amend_ the list of folders that should be
configured as "writable roots" by the sandbox used in `full-auto` mode.
Values that are passed as relative paths will be resolved to absolute
paths.

Incidentally, this required updating a number of the `agent*.test.ts`
files: it feels like some of the setup logic across those tests could be
consolidated.

In my testing, it seems that this might be slightly out of distribution
for the model, as I had to explicitly tell it to run `apply_patch` and
that it had the permissions to write those files (initially, it just
showed me a diff and told me to apply it myself). Nevertheless, I think
this is a good starting point.

codex-cli/src/app.tsx

@@ -22,6 +22,7 @@ type Props = {
   imagePaths?: Array<string>;
   rollout?: AppRollout;
   approvalPolicy: ApprovalPolicy;
+  additionalWritableRoots: ReadonlyArray<string>;
   fullStdout: boolean;
 };
 
@@ -31,6 +32,7 @@ export default function App({
   rollout,
   imagePaths,
   approvalPolicy,
+  additionalWritableRoots,
   fullStdout,
 }: Props): JSX.Element {
   const app = useApp();
@@ -97,6 +99,7 @@ export default function App({
       prompt={prompt}
       imagePaths={imagePaths}
       approvalPolicy={approvalPolicy}
+      additionalWritableRoots={additionalWritableRoots}
       fullStdout={fullStdout}
     />
   );