feat: stack-aware AI quality checklist (PROJECT_STACK)

Summary

The AI quality checklist was React-Native-only, so web / backend code in a monorepo was scored against irrelevant rules. This makes the checklist stack-aware.

Changes

• PROJECT_STACK config (rn | web | backend | generic, default rn → no behavior change for existing projects).
• ai-checklist.md — 4 dimensions stay CORE (all stacks); framework items tagged [stack:rn] / [stack:rn|web]; new STACK PROFILES section adds web (a11y, SSR/CSR, next/image, env exposure, XSS, bundle) and backend (input validation, injection, N+1, transactions, error→HTTP, secrets, auth/rate-limit). generic = CORE only.
• /check-conventions, /smart-refactor, /review-pr — apply CORE + the profile matching $PROJECT_STACK.

Test plan

• /check-conventions on an RN file with PROJECT_STACK=rn → same as before
• /check-conventions on a Next.js file with PROJECT_STACK=web → web rules apply, no FlatList/AsyncStorage noise
• PROJECT_STACK=backend → injection / N+1 / transaction rules apply, UI items skipped