test(import): add 22 comprehensive tests for ralph_import.sh
Add integration tests for PRD to Ralph format conversion:
- File format support tests (.md, .txt, .json)
- Output file creation tests (PROMPT.md, @fix_plan.md, specs/requirements.md)
- Project naming tests (custom names, auto-detection from filename)
- Error handling tests (missing files, dependencies, conversion failures)
- Help and usage tests
- Full workflow integration tests
- Edge case handling (hyphens, uppercase, subdirectory paths)
Test infrastructure:
- Mock ralph-setup command using PATH manipulation
- Mock Claude Code CLI for isolated conversion testing
- Added create_sample_prd_txt() fixture helper
Test count: 223 (up from 201)
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