feat: read approvalMode from config file (#298)

This PR implements support for reading the approvalMode setting from the
user's config file (`~/.codex/config.json` or `~/.codex/config.yaml`),
allowing users to set a persistent default approval mode without needing
to specify command-line flags for each session.

Changes:
- Added approvalMode to the AppConfig type in config.ts
- Updated loadConfig() to read the approval mode from the config file
- Modified saveConfig() to persist the approval mode setting
- Updated CLI logic to respect the config-defined approval mode (while
maintaining CLI flag priority)
- Added comprehensive tests for approval mode config functionality
- Updated README to document the new config option in both YAML and JSON
formats
- additions to `.gitignore` for other CLI tools

Motivation:
As a user who regularly works with CLI-tools, I found it odd to have to
alias this with the command flags I wanted when `approvalMode` simply
wasn't being parsed even though it was an optional prop in `config.ts`.
This change allows me (and other users) to set the preference once in
the config file, streamlining daily usage while maintaining the ability
to override via command-line flags when needed.

Testing:
I've added a new test case loads and saves approvalMode correctly that
verifies:
- Reading the approvalMode from the config file works correctly
- Saving the approvalMode to the config file works as expected
- The value persists through load/save operations

All tests related to the implementation are passing.

codex-cli/tests/config.test.tsx

@@ -1,6 +1,7 @@
 import type * as fsType from "fs";
 
 import { loadConfig, saveConfig } from "../src/utils/config.js"; // parent import first
+import { AutoApprovalMode } from "../src/utils/auto-approval-mode.js";
 import { tmpdir } from "os";
 import { join } from "path";
 import { test, expect, beforeEach, afterEach, vi } from "vitest";
@@ -107,3 +108,43 @@ test("loads user instructions + project doc when codex.md is present", () => {
     userInstr + "\n\n--- project-doc ---\n\n" + projectDoc,
   );
 });
+
+test("loads and saves approvalMode correctly", () => {
+  // Setup config with approvalMode
+  memfs[testConfigPath] = JSON.stringify(
+    {
+      model: "mymodel",
+      approvalMode: AutoApprovalMode.AUTO_EDIT,
+    },
+    null,
+    2,
+  );
+  memfs[testInstructionsPath] = "test instructions";
+
+  // Load config and verify approvalMode
+  const loadedConfig = loadConfig(testConfigPath, testInstructionsPath, {
+    disableProjectDoc: true,
+  });
+
+  // Check approvalMode was loaded correctly
+  expect(loadedConfig.approvalMode).toBe(AutoApprovalMode.AUTO_EDIT);
+
+  // Modify approvalMode and save
+  const updatedConfig = {
+    ...loadedConfig,
+    approvalMode: AutoApprovalMode.FULL_AUTO,
+  };
+
+  saveConfig(updatedConfig, testConfigPath, testInstructionsPath);
+
+  // Verify saved config contains updated approvalMode
+  expect(memfs[testConfigPath]).toContain(
+    `"approvalMode": "${AutoApprovalMode.FULL_AUTO}"`,
+  );
+
+  // Load again and verify updated value
+  const reloadedConfig = loadConfig(testConfigPath, testInstructionsPath, {
+    disableProjectDoc: true,
+  });
+  expect(reloadedConfig.approvalMode).toBe(AutoApprovalMode.FULL_AUTO);
+});