chore: deprecate --ai flag in favor of --integration on specify init (#2218)

* chore: deprecate --ai flag in favor of --integration on specify init

- Adds deprecation warning when --ai is used
- Shows equivalent --integration command replacement
- Handles generic integration with --commands-dir mapping
- Adds comprehensive test coverage for deprecation behavior
- Warning displays as prominent red panel above Next Steps
- --ai flag continues to function (non-breaking change)

Fixes #2169

* Address PR review feedback for issue #2169

- Use existing strip_ansi helper from conftest instead of duplicating ANSI escape pattern
- Properly escape ai_commands_dir with shlex.quote() to handle paths with spaces
- Add shlex import to support proper command-line argument escaping