Update golangci-lint to v2.12.2 with canonical config

- Add canonical .golangci.yml (v2 schema, default: all, project
  thresholds for lll/funlen/cyclop/dupl)
- Bump golangci-lint pins from v2.0.2 to v2.12.2 in Makefile
  (go install, new /v2 module path) and Dockerfile (tagged+digest
  Debian image pin)
- Fix all lint findings surfaced by the new linter set across
  cmd/mfer, internal/bork, internal/cli, internal/log, and mfer:
  static sentinel errors (err113), context-aware HTTP and exec
  (noctx), guarded integer conversions and stricter permissions
  (gosec), named constants (mnd, goconst), function decomposition
  (funlen, cyclop, gocognit, nestif), declaration ordering
  (funcorder), t.Parallel/t.TempDir/t.Setenv adoption in tests
  (paralleltest, usetesting), protobuf getters (protogetter), plus
  formatting and style cleanups (wsl_v5, nlreturn, lll, revive,
  testifylint, and others)
- Serialize CLI runs in tests behind a mutex so parallel tests do
  not cross-wire the process-global logger's captured output

internal/log/log_test.go

@@ -1,12 +1,12 @@
-package log
+package log_test
 
 import (
 	"testing"
 
-	"github.com/stretchr/testify/assert"
+	"sneak.berlin/go/mfer/internal/log"
 )
 
 func TestBuild(t *testing.T) {
-	Init()
-	assert.True(t, true)
+	t.Parallel()
+	log.Init()
 }