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/tests/agent-invalid-request-error.test.ts

@@ -58,6 +58,7 @@ describe("AgentLoop – invalid request / 4xx errors", () => {
       model: "any",
       instructions: "",
       approvalPolicy: { mode: "auto" } as any,
+      additionalWritableRoots: [],
       onItem: (i) => received.push(i),
       onLoading: () => {},
       getCommandConfirmation: async () => ({ review: "yes" } as any),