docs: add comprehensive deployment pipeline architecture and optimization plan

- Add deployment-pipeline-architecture.md (14k+ words, detailed design)
  - Three-tier test strategy (fast/deploy/E2E)
  - Branch strategy (main/develop/feature/ci-experimental)
  - Cost optimization (50-60% savings)
  - Rollback strategy (< 1 min automated)
  - Quality gates and monitoring

- Add deployment-pipeline-diagram.md (visual diagrams)
  - Current vs proposed architecture comparison
  - Branch flow and test execution decision trees
  - Test pyramid and coverage breakdown
  - Monitoring dashboard templates

- Add deployment-pipeline-implementation-checklist.md
  - Phase 1: Critical path (Week 1) - unblock staging
  - Phase 2: Optimization (Week 2) - reduce costs
  - Phase 3: Advanced features (Week 3-4)
  - Validation checklist and success metrics

- Add deployment-pipeline-summary.md (executive summary)
  - Problem statement and business impact
  - Solution overview and benefits
  - ROI calculation (558% first year)
  - Approval section and next steps

Key Improvements:
- Staging deploys in < 5 min (from 20+ min) - 75% faster
- E2E tests run post-deployment (non-blocking)
- GitHub Actions cost: $26/month (from $80) - 67% savings
- Developer time saved: 10 hours/week
- Automated rollback capability

Related to ongoing CI/CD improvements and staging deployment issues

Author: frankbria