feat: gate major narrative stage promotions

[barretlee]

Summary

• add an evidence-gated, low-frequency narrative stage promotion pipeline
• require DeepSeek V4 Pro thinking/high effort plus local Event, Evidence, Source, confidence, novelty, and timeline checks
• create/update a dedicated stage:milestone Issue before persisting a versioned stage record
• keep stage assessment failures advisory so deterministic snapshot commits and Pages updates continue
• include narrative changes in public-content fingerprinting and document the workflow in an SDD package

Quality gates

• npm run check
• npm run build
• 49 test files / 309 tests
• all workflow YAML parsed
• new stage workflow Bash blocks pass bash -n
• real V4 Pro smoke on the current strongest candidate returned hold; no stage or Issue was created