fix: add explicit ZCode model bootstrap

[rldyourmnd]

Summary

• add the explicit provider/model bootstrap required by ZCode CLI 0.15.2
• keep OAuth credentials runtime-owned and separate optional API-key providers under custom identities
• fail plan/apply on incomplete model provider contracts or reserved custom identities
• release nddev-zcode-app 2.1.2

Verification

• private nddev-harnesses targeted suite: 60 passed
• private fast lane: PASS
• all three setup plans: PASS
• isolated CLI 0.15.2 adapter initialization: passed beyond model_config_missing