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/src/utils/config.ts

@@ -77,6 +77,7 @@ export type AppConfig = {
   apiKey?: string;
   model: string;
   instructions: string;
+  approvalMode?: AutoApprovalMode;
   fullAutoErrorMode?: FullAutoErrorMode;
   memory?: MemoryConfig;
   /** Whether to enable desktop notifications for responses */
@@ -275,6 +276,7 @@ export const loadConfig = (
         : DEFAULT_AGENTIC_MODEL),
     instructions: combinedInstructions,
     notify: storedConfig.notify === true,
+    approvalMode: storedConfig.approvalMode,
     safeCommands: storedConfig.safeCommands ?? [],
   };
 
@@ -391,6 +393,7 @@ export const saveConfig = (
   // Create the config object to save
   const configToSave: StoredConfig = {
     model: config.model,
+    approvalMode: config.approvalMode,
   };
 
   // Add history settings if they exist