chore: checkpoint phase 3 sync-config rewrite

[georgewrmarshall]

Summary

• add a parallel typed phase 3 sync-config rewrite at pipeline/sync-config.ts
• split stable rewrite inputs into config.static.json
• document the current metrics behavior and phase 3 cutover risk in PHASE-3-NOTES.md
• keep the production sync path on scripts/sync-config.js and expose the rewrite through sync-config:next* scripts only

Why this is draft

The rewrite is not ready for cutover yet. Validation showed that switching the main pipeline to source-annotated deprecated components would change the current metrics contract and drop many currently tracked deprecated entries from config.json.

Context

• notes captured in PHASE-3-NOTES.md
• missing deprecation-message backfill issue: Backfill missing deprecation messages for tracked deprecated components #49

Validation

• yarn typecheck
• yarn node scripts/sync-config.js --dry-run --skip-update
• rewrite validation run separately and intentionally left as a parallel path because it still diverges from the current config