chore(lint): adopt the canonical .golangci.yml from infra/ci-templates

Canonical lives at infra ci-templates/golangci.yml, distributed by the
infra#309 pattern: byte-identical copies, checksum drift gate in infra. Do
not edit this file here -- edit the canonical and run `task ci:sync`.

Verified locally before pushing, not pushed-and-hoped: golangci-lint v2.11.4
against a Go 1.26.1 toolchain (matching CI) reports 0 issues here.

Also applies gofumpt, which this config enables and the golangci-lint v2
default set does not check at all. That gap is why the drift was invisible:
earlier passes ran gofmt, which is weaker. Mechanical, deterministic,
build+vet verified.

ingestion/internal/startupretry/startupretry_test.go

@@ -18,7 +18,6 @@ func TestDo_ReturnsImmediatelyOnFirstSuccess(t *testing.T) {
 		calls++
 		return "ready", nil
 	})
-
 	if err != nil {
 		t.Fatalf("Do: %v", err)
 	}
@@ -41,7 +40,6 @@ func TestDo_SurvivesTransientFailuresThenSucceeds(t *testing.T) {
 		}
 		return "ready", nil
 	})
-
 	if err != nil {
 		t.Fatalf("Do returned %v — a transient 502 must not be fatal", err)
 	}