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/components/chat/terminal-chat.tsx

@@ -37,6 +37,7 @@ type Props = {
   prompt?: string;
   imagePaths?: Array<string>;
   approvalPolicy: ApprovalPolicy;
+  additionalWritableRoots: ReadonlyArray<string>;
   fullStdout: boolean;
 };
 
@@ -122,6 +123,7 @@ export default function TerminalChat({
   prompt: _initialPrompt,
   imagePaths: _initialImagePaths,
   approvalPolicy: initialApprovalPolicy,
+  additionalWritableRoots,
   fullStdout,
 }: Props): React.ReactElement {
   const [model, setModel] = useState<string>(config.model);
@@ -183,6 +185,7 @@ export default function TerminalChat({
       config,
       instructions: config.instructions,
       approvalPolicy,
+      additionalWritableRoots,
       onLastResponseId: setLastResponseId,
       onItem: (item) => {
         log(`onItem: ${JSON.stringify(item)}`);
@@ -248,7 +251,13 @@ export default function TerminalChat({
       agentRef.current = undefined;
       forceUpdate(); // re‑render after teardown too
     };
-  }, [model, config, approvalPolicy, requestConfirmation]);
+  }, [
+    model,
+    config,
+    approvalPolicy,
+    requestConfirmation,
+    additionalWritableRoots,
+  ]);
 
   // whenever loading starts/stops, reset or start a timer — but pause the
   // timer while a confirmation overlay is displayed so we don't trigger a