Add Codex Prompt Center

[blancmathis]

Summary

• add a global Context Hub prompt editor driven by a compatible Codex runtime catalog
• persist exact private overrides outside project configuration
• verify per-process runtime-loaded state with strict publication receipts and immutable snapshots
• expose protected and server-owned prompt families as read-only metadata

Validation

• npm test: 313 passed
• node bin/context-room.mjs doctor --root .: OK (11 pre-existing low external-context notices)
• npm run package:privacy: OK (46 files)
• npm pack --dry-run: OK
• git diff --check: OK