fix: skip transient codex tmp files in session backup

[skulidropek]

Summary

• skip .codex/tmp/** when collecting session backup files so transient lock files do not enter the snapshot
• export the session backup helpers behind a require.main === module guard for direct regression testing
• add a regression test that proves .codex/tmp is excluded while persistent session files are still preserved

Verification

• node --check scripts/session-backup-gist.js
• pnpm --filter ./packages/app exec vitest run tests/hooks/session-backup-gist.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

Notes

• pnpm --filter ./packages/app lint:tests still fails on unrelated pre-existing errors elsewhere under packages/app/tests; this change adds a file that passes targeted linting and the full lint:effect profile.

[skulidropek]

Closing this in favor of #205. The useful testability pieces from this PR (require.main guard, exported helpers, and direct session-backup regression coverage) have been integrated on top of the broader tmp-skip fix there, so we should keep a single review path.