test(scripts): cover session backup tmp filtering

[skulidropek]

Summary

• make scripts/session-backup-gist.js directly testable via a require.main === module guard and explicit helper exports
• add a typed declaration for those exported helpers so packages/app tests can import the script cleanly
• add direct regression coverage proving session backup skips tmp directories while preserving persistent session files

Context

• #205 already merged the broader runtime fix that skips tmp directories in recursive scans
• this follow-up keeps the useful testability pieces from closed #204, adapted to the broader semantics now on main

Validation

• node --check scripts/session-backup-gist.js
• pnpm --filter ./packages/app exec vitest run tests/hooks/session-backup-gist.test.ts tests/hooks/pre-commit-ai-dirs.test.ts
• pnpm --filter ./packages/app exec eslint tests/hooks/session-backup-gist.test.ts --ext .ts
• pnpm --filter ./packages/app lint:effect
• pnpm --filter ./packages/app typecheck