ah-nyl.9 refit: testify + culpa in domain/config/store/vikunja Past Stand

The repo conventions changed after these four packages landed (SPEC SS13 now): tests must use testify (github.com/stretchr/testify require/assert), and errors must be constructed/wrapped via go.bigb.es/auxilia/culpa (New/Errorf/Wrap/Wrapf; keep stdlib errors.New sentinels where callers use errors.Is). Both deps are already in go.mod.

Scope — exactly these packages, which are DONE and committed: internal/domain, internal/config, internal/store, internal/vikunja. Do NOT touch internal/runner, internal/reconcile, cmd/ (siblings are working there right now), go.mod/go.sum, docs/, .beads/.

Work:
1. Convert all *_test.go in the four packages to testify: require.* for fatal paths (setup, errors), assert.* for value checks where the test can meaningfully continue. Preserve every existing test case and its semantics — this is a mechanical style conversion, not a rewrite; keep table-test structures intact.
2. Convert error construction in non-test code of the four packages from fmt.Errorf to culpa equivalents (fmt.Errorf("...: %w", err) -> culpa.Wrapf/Wrap; fmt.Errorf without %w -> culpa.Errorf/New). Keep exported sentinels (ErrDuplicateRun, ErrRunNotFound) as-is so errors.Is keeps working; culpa-wrapped returns must still satisfy errors.Is against those sentinels where they did before (culpa supports errors.Is chains — verify with the existing duplicate-run test). Multi-error collection in config validation may stay errors.Join or move to culpa.Join — pick what keeps the error text readable and the tests passing with minimal churn.
3. No signature changes, no behavior changes, no coverage loss. Per-package verify: go build/vet/test for ./internal/domain/... ./internal/config/... ./internal/store/... ./internal/vikunja/... (these four only; NEVER ./...).

Commit per package or as one commit: 'refit: testify + culpa in <pkgs>' staging only the four package dirs. Hooks bypassed (git -c core.hooksPath=/tmp/nohooks commit). Do NOT push.