fix: preserve user settings when enabling/disabling hooks

[Damon-GSY]

Bug Description

This PR fixes a critical bug where cn off destroys all user settings in settings.json.

The Problem

Before this fix, running cn off would:

1. Overwrite the user's preferred model (e.g., sonnet) to opus
2. Delete all permission rules
3. Delete all environment variables
4. Delete any other custom settings

Example:

// User's original settings.json
{
  "model": "sonnet",
  "permissions": {
    "allow": ["Bash(npm run*)", "Bash(pytest*)"]
  },
  "env": {
    "MY_API_KEY": "secret"
  }
}

After cn off (buggy behavior):

{
  "model": "opus"  // Everything else is GONE!
}

The Fix

Now cn off only removes the hooks section while preserving all other settings:

// After cn off (fixed behavior)
{
  "model": "sonnet",  // Preserved!
  "permissions": {
    "allow": ["Bash(npm run*)", "Bash(pytest*)"]  // Preserved!
  },
  "env": {
    "MY_API_KEY": "secret"  // Preserved!
  }
}

Changes

Function	Before	After
enable_hooks_in_settings()	Hardcoded "model": "opus"	Preserves all existing settings
disable_hooks_in_settings()	Rewrote entire file	Only removes hooks key
disable_gemini_hooks()	Rewrote entire file	Only removes code-notify hooks

Implementation Details

• Uses jq (preferred) for JSON manipulation
• Falls back to python3 if jq is not available
• Warns user if neither tool is available (safer than corrupting config)

Testing

Added tests/test-config-preservation.sh with 4 test cases:

Test	Description	Status
Test 1	enable_hooks preserves existing model	✅ PASS
Test 2	disable_hooks preserves other settings	✅ PASS
Test 3	Works with no existing config	✅ PASS
Test 4	Python fallback works correctly	✅ PASS

Run tests:

./tests/test-config-preservation.sh

Documentation

Added docs/BUG_FIX_CONFIG_PRESERVATION.md with:

• Detailed before/after examples
• Technical explanation of the fix
• Migration notes

Files Changed

File	Changes
lib/code-notify/core/config.sh	Fixed 3 functions to preserve user settings
tests/test-config-preservation.sh	New test file
docs/BUG_FIX_CONFIG_PRESERVATION.md	New documentation

Checklist

• Bug fix (non-breaking change which fixes an issue)
• I have performed a self-review of my own code
• I have added tests that prove my fix is effective
• I have updated the documentation