chore(dagger): align Dagger versions to v0.21.4 and add lint (#544)

## 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

.pre-commit-config.yaml

@@ -53,3 +53,9 @@ repos:
         entry: bash -c 'cd "$(git rev-parse --show-toplevel)" && nix develop --command task check-ci-images'
         pass_filenames: false
         files: ^(ci/main\.go|\.fvmrc)$
+      - id: dagger-versions-aligned
+        name: verify Dagger version is consistent across dagger.json, flake.nix, Dockerfile and DAGGER.md
+        language: system
+        entry: bash -c 'cd "$(git rev-parse --show-toplevel)" && scripts/check_dagger_versions.sh'
+        pass_filenames: false
+        files: ^(ci/dagger\.json|flake\.nix|\.forgejo/Dockerfile|DAGGER\.md)$