refactor: extract rollback logic to rollback-stacks.sh (Phase 5)

Extract rollback and rollback health verification from inline heredocs to modular scripts:

Changes:
- Create scripts/deployment/rollback-stacks.sh (495 lines)
  - Extracts rollback logic with stack discovery and parallel execution
  - Validates previous SHA format before rollback attempt
  - Dynamically discovers stacks from previous commit structure
  - Implements parallel rollback with exit code file-based error detection
  - Outputs discovered_rollback_stacks for health verification
  - Uses ssh_retry for reliable remote execution

- Update "Rollback to Previous Version" step (line 594)
  - Replace 430+ line heredoc with 19-line script call
  - Pass all required parameters (SHA, timeouts, secrets)
  - Capture and parse script output

- Update "Verify Rollback Health" step (line 622)
  - Replace 205-line heredoc with 67-line call to health-check.sh
  - Reuse existing health-check.sh for rollback verification
  - Convert discovered stacks from null-delimited to space-delimited
  - Add rollback_ prefix to outputs for workflow compatibility

Impact:
- Workflow reduced from 1,392 → 851 lines (39% reduction)
- Overall reduction from original: 2,548 → 851 lines (67% reduction)
- Eliminates duplicate health checking logic
- Improves maintainability with modular scripts
- Validates with actionlint (passes)

Phase 5 complete ✅

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: owine