Introduces a new `lint` Makefile target that checks for golangci-lint installation,
runs the linter to match CI pipeline, and updates the help message accordingly.
- Introduce tests for chat history file handling, loading/saving, and error cases in cmd/chat_test.go
- Add tests for removeLastModifiedComments in cmd/edit_helper_test.go
- Add comprehensive tests for CleanCodeResponse in internal/grok/cleancode_test.go
- Update Makefile to centralize build artifacts in build/ directory for coverage reports and binary
- Adjust .gitignore to ignore chat_history.json and remove obsolete coverage file entries
- Introduced Makefile with targets for testing (all, coverage, agent-specific), building, installing, cleaning, and help
- Added unit and integration tests for agent command, edit command, and CleanCodeResponse function
- Refactored CleanCodeResponse to use regex for robust markdown fence removal in agent and client modules
- Ensured tests cover code cleaning, plan generation placeholders, and file editing functionality
