Add support for .ticket-config.override.yaml configuration override

[mkanakana]

Summary

Add support for .ticket-config.override.yaml file to allow local configuration overrides without modifying the main configuration file. This enables personalized workflows for different developers and environments.

Key Features

• Optional override file: System works normally without the override file
• Precedence handling: Override values take precedence over main config
• New field support: Can add completely new configuration fields
• Auto gitignore: Automatically added to .gitignore during init
• Comprehensive testing: 100+ test cases covering all scenarios

Implementation Details

• Added load_config_with_override() function in lib/utils.sh
• Updated all config loading points in src/ticket.sh (8 locations)
• Enhanced init command to add override file to .gitignore
• Maintains full backward compatibility

Use Cases

• Developer-specific settings: Different branch prefixes, directories, or messages
• Environment-specific config: Disable auto-push for testing environments
• Team customization: Team members can have personalized workflows
• Local development: Override settings without affecting shared config

Example Usage

Create .ticket-config.override.yaml:

# Override specific values from main config
tickets_dir: "my-custom-tickets"
auto_push: false
start_success_message: |
  Custom message for this environment

Test Plan

• All existing tests pass (143/144)
• New comprehensive test suite covers:
  • Basic override functionality
  • Precedence rules (override wins over main config)
  • Optional behavior (works without override file)
  • Error handling for malformed files
  • Support for both .yaml and .yml main config files
• Manual testing in different environments
• Documentation updated (README, spec, DEV.md)

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