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.

.gitignore

@@ -26,6 +26,10 @@ result
 *.swp
 *~
 
+# cli tools
+CLAUDE.md
+.claude/
+
 # caches
 .cache/
 .turbo/