Add Ghost drift loop contract

[gnahCnayR]

:bufobot: Ryan's Agent speaking...

Summary

• add a Ghost-owned ghost drift command surface with status and check
• add explicit opt-in .ghost/config.yml design loop config (enabled, mode: off|advisory|required) defaulting off
• make drift/track/ack flows package-aware for canonical .ghost/fingerprint/ packages while preserving legacy .ghost/fingerprint.md behavior
• harden drift checks against ledger fingerprint mismatches, npm-tracked canonical packages, manifest-path references, and digest-only canonical package changes

Validation

• pnpm test
• npm_config_registry=https://global.block-artifacts.com/artifactory/api/npm/square-npm/ pnpm check
• sq agents review --local --codex
• SQ_DESIGN_EXPERIMENTAL=1 npm run smoke in squareup/sq-design-market-react-scenarios
• direct scenario checks against 01-market-react-aligned, 02-custom-css-drift, and 03-market-react-covered-monorepo --local apps/admin/.ghost
• temp scenario matrix for design-loop status opt-in, ghost track bootstrap, covered drift, and post-ack mutation detection

[nahiyankhan]

feel free to merge after fixing the ci! let me know if something else borked though