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/utils/agent/agent-loop.ts

@@ -45,6 +45,9 @@ type AgentLoopParams = {
   onItem: (item: ResponseItem) => void;
   onLoading: (loading: boolean) => void;
 
+  /** Extra writable roots to use with sandbox execution. */
+  additionalWritableRoots: ReadonlyArray<string>;
+
   /** Called when the command is not auto-approved to request explicit user review. */
   getCommandConfirmation: (
     command: Array<string>,
@@ -58,6 +61,7 @@ export class AgentLoop {
   private instructions?: string;
   private approvalPolicy: ApprovalPolicy;
   private config: AppConfig;
+  private additionalWritableRoots: ReadonlyArray<string>;
 
   // Using `InstanceType<typeof OpenAI>` sidesteps typing issues with the OpenAI package under
   // the TS 5+ `moduleResolution=bundler` setup. OpenAI client instance. We keep the concrete
@@ -213,6 +217,7 @@ export class AgentLoop {
     onLoading,
     getCommandConfirmation,
     onLastResponseId,
+    additionalWritableRoots,
   }: AgentLoopParams & { config?: AppConfig }) {
     this.model = model;
     this.instructions = instructions;
@@ -229,6 +234,7 @@ export class AgentLoop {
         model,
         instructions: instructions ?? "",
       } as AppConfig);
+    this.additionalWritableRoots = additionalWritableRoots;
     this.onItem = onItem;
     this.onLoading = onLoading;
     this.getCommandConfirmation = getCommandConfirmation;
@@ -358,6 +364,7 @@ export class AgentLoop {
         args,
         this.config,
         this.approvalPolicy,
+        this.additionalWritableRoots,
         this.getCommandConfirmation,
         this.execAbortController?.signal,
       );