[v6] Upgrade Script: Pipeline YAML Migration

[jessehouwing]

Scope: Implement scripts/upgrade-pipelines.ts converting v4/v5 steps to unified v6 task.
Acceptance Criteria:

• CLI flags: --dry-run, --in-place, --backup, --report.
• Mapping table covers ≥90% legacy inputs.
• Generates diff summary.
  Tasks:
• Define mapping table JSON.
• Parse YAML and identify task sequences.
• Group steps -> composite config.
• Output new YAML + report.
  Labels: enhancement, help wanted