fix: fail fast in CI — parallel hygiene/layer checks, no spurious retries (#350)

## Summary

Closes #349

Two bugs prevented `check-dagger` from failing fast when checks failed:

- **Hygiene + Layers checked sequentially** — they are cheap structural checks with no dependency on each other. Running them in parallel (`errgroup.Group`) means failures are reported sooner.
- **Spurious retries from `errgroup.WithContext`** — the backend and integration tests previously shared a derived context via `errgroup.WithContext`. When one test failed, the context was cancelled, causing the sibling test to emit `"context canceled"` in Dagger's `--progress=plain` output. The `retry_dagger` function in `Taskfile.yml` matched that string as a transient network error and re-ran the entire pipeline up to 3 times — a real test failure could take 30+ minutes to be reported instead of ~10.

**Fix in `ci/main.go`:**
- Hygiene + layers now run in parallel with `errgroup.Group`
- Backend + integration tests now use `errgroup.Group` (no shared cancel context), so a failure in one does not emit `"context canceled"` for the other

**Fix in `Taskfile.yml`:**
- Removed `context canceled` from the `retry_dagger` grep pattern; the remaining patterns (`connection reset`, `context deadline exceeded`, `connection refused`, `invalid return status code`) still cover genuine network/engine transients

## Test plan

- [ ] Confirm the Forgejo CI run completes and, when a check fails, it fails fast (no 3× retry loop in logs)
- [ ] Verify `task check-dagger` still retries on actual connection errors

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Thomas SharedInbox <sharedinbox@thomas-guettler.de>
Co-authored-by: guettli <guettli@noreply.codeberg.org>
Reviewed-on: https://codeberg.org/guettli/sharedinbox/pulls/350

Taskfile.yml

@@ -294,7 +294,7 @@ tasks:
           for attempt in 1 2 3; do
             run_dagger "$@" && return 0
             RC=$?
-            if [ "$attempt" -lt 3 ] && { grep -qE "connection reset|context canceled|context deadline exceeded|connection refused|invalid return status code" "$DAGGER_OUT" || [ "$RC" -eq 2 ]; }; then
+            if [ "$attempt" -lt 3 ] && { grep -qE "connection reset|context deadline exceeded|connection refused|invalid return status code" "$DAGGER_OUT" || [ "$RC" -eq 2 ]; }; then
               echo "$(_ts) dagger: network error on attempt $attempt/3, retrying..." >&2
             elif [ "$attempt" -lt 3 ] && grep -q "No space left on device" "$DAGGER_OUT"; then
               echo "$(_ts) dagger: disk space error on attempt $attempt/3, pruning Dagger cache..." >&2