feat: add automated rollback script (resolves #61)

[maxrantil]

Summary

Implements automated rollback script to restore dotfiles from backup directories created by install.sh. This addresses the critical production confidence gap identified in Issue #61.

Changes

• rollback.sh: New automated rollback script with comprehensive features

  • Automatic detection of latest backup directory
  • Interactive confirmation prompt (can be skipped with -y)
  • Dry-run mode (--dry-run) for previewing changes
  • Symlink removal from all standard locations (including ZDOTDIR)
  • File restoration with permission preservation
  • Automatic cleanup of empty backup directories
  • Hidden file support using dotglob
  • Security hardening: Backup directory validation, input sanitization, TOCTOU mitigation

• tests/rollback-test.sh: Comprehensive test suite

  • 11 test scenarios covering all functionality
  • 13 passing assertions
  • Tests include: backup discovery, error handling (no backup, empty backup, invalid format), non-interactive mode, symlink removal, file content preservation, permission preservation, and dry-run mode

• README.md: Documentation updates

  • Added "Rollback" section with usage examples
  • Updated test coverage list to include rollback functionality
  • Updated last modified date

Test Results

✅ Rollback tests: 11/11 tests passed (13 assertions)
✅ Pre-commit hooks: All checks passed
✅ Security validation: All HIGH severity issues addressed
✅ Production readiness: All blockers resolved

Security & Production Readiness Validation

Comprehensive validation performed by security-validator and devops-deployment-agent:

Security Improvements Implemented:

• ✅ Backup directory name format validation (prevents malicious directories)
• ✅ Empty backup directory validation (prevents broken system state)
• ✅ ZDOTDIR input validation (prevents command injection)
• ✅ TOCTOU mitigation in symlink removal (double-check pattern)
• ✅ Enhanced error handling with explicit failure messages

Production Readiness Assessment:

• Overall Score: 4.2/5.0 (Ready for Production)
• Security Rating: 3.5/5.0 (Adequate for single-user context)
• Reliability: 4.5/5.0 - All tests pass, robust backup discovery
• Safety: 4.0/5.0 - Confirmation prompts, dry-run mode, validation
• Testing: 4.5/5.0 - Comprehensive automated test suite
• Documentation: 4.0/5.0 - Clear usage examples, help text

Issues Addressed:

• Fixed 2 production blockers (empty backup validation, shell formatting)
• Resolved 3 HIGH severity security issues
• Added 2 new test cases for edge case coverage

Testing Instructions

# Run rollback-specific tests
./tests/rollback-test.sh

# Test in Docker environment
./tests/docker-test.sh

# Manual testing
./install.sh           # Create installation
./rollback.sh --dry-run  # Preview rollback
./rollback.sh -y       # Perform rollback

Implementation Details

TDD Approach:

1. ✅ RED: Created failing tests first
2. ✅ GREEN: Implemented minimal working solution
3. ✅ REFACTOR: Cleaned up code while maintaining tests
4. ✅ HARDEN: Added security validations based on expert review

Key Technical Decisions:

• Uses shopt -s dotglob nullglob to handle hidden files in backup
• Respects ZDOTDIR configuration from .zprofile
• Preserves file permissions using mv instead of cp
• Removes symlinks before restoration to avoid conflicts
• Cleans up backup directory only if empty after restoration
• Validates backup directory format and contents before proceeding
• Sanitizes ZDOTDIR input to prevent command injection
• Uses double-check pattern to mitigate TOCTOU vulnerabilities

Security & Safety

• Confirmation prompt by default prevents accidental data loss
• Dry-run mode allows safe preview of changes
• Error handling for missing, empty, and malformed backup directories
• Input validation for all external data sources
• TOCTOU race condition mitigation in symlink operations
• No destructive operations without user consent (except with -y flag)

References

• Issue Add automated rollback script for dotfiles installation #61: Original feature request
• Validation performed per CLAUDE.md Section 2 requirements
• Security findings documented in session handoff

Resolves #61