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.

README.md

@@ -281,11 +281,12 @@ pnpm link
 
 ## Configuration
 
-Codex looks for config files in **`~/.codex/`**.
+Codex looks for config files in **`~/.codex/`** (either YAML or JSON format).
 
 ```yaml
 # ~/.codex/config.yaml
 model: o4-mini # Default model
+approvalMode: suggest # or auto-edit, full-auto
 fullAutoErrorMode: ask-user # or ignore-and-continue
 notify: true # Enable desktop notifications for responses
 safeCommands:
@@ -293,6 +294,16 @@ safeCommands:
   - yarn lint # Automatically approve yarn lint
 ```
 
+```json
+// ~/.codex/config.json
+{
+  "model": "o4-mini",
+  "approvalMode": "suggest",
+  "fullAutoErrorMode": "ask-user",
+  "notify": true
+}
+```
+
 You can also define custom instructions:
 
 ```yaml