## Summary
Closes#542.
- Bumped `ci/dagger.json` `engineVersion`, the Forgejo runner Dockerfile (`.forgejo/Dockerfile`), and the example `dagger-engine.service` unit in `DAGGER.md` from `0.20.8` -> `0.21.4` so they match the running engine and the CLI already pinned by `flake.nix`.
- Added `scripts/check_dagger_versions.sh` which parses the four pinned references and fails if any drift apart.
- Wired the lint into `Taskfile.yml` (`task check-dagger-versions`) and `.pre-commit-config.yaml` (triggered when any of the four pinned files change).
## Verification
- `./scripts/check_dagger_versions.sh` -> passes, all four references at `v0.21.4`.
- Temporarily edited `ci/dagger.json` to `v0.21.3` and re-ran the script: exits non-zero with a clear "out of sync" error.
Generated with Claude Code.
Reviewed-on: https://codeberg.org/guettli/sharedinbox/pulls/544
