build: update golangci-lint to v2.12.2 with canonical config

Add the canonical .golangci.yml (v2 schema, all linters enabled with a
small documented disable list) and pin the Dockerfile lint stage to
golangci/golangci-lint:v2.12.2 by tag and digest, replacing the old
v1.64.8 digest-only pin.

Fix all findings surfaced by the v1 to v2 jump without changing any
exported signatures or behavior:

- add package and exported-symbol doc comments (revive)
- rename unused handler parameters to underscore (revive)
- check or explicitly discard error returns (errcheck, errchkjson)
- wrap errors with %w instead of %v (err113)
- use http.NewRequestWithContext instead of http.Post (noctx)
- replace fmt.Println with fmt.Fprintln(os.Stdout, ...) (forbidigo)
- name magic numbers as constants (mnd)
- add explicit slog.LevelDebug case (exhaustive)
- interface{} to any (modernize)
- move tests to the simplelog_test package (testpackage) and add
  t.Parallel() (paralleltest)
- move NewWebhookHandler above its methods (funcorder)
- whitespace, line-length, and blank-line fixes (wsl_v5, whitespace,
  nlreturn, lll, embeddedstructfieldcheck)
- nolint with justification for the intentional init/global design
  (gochecknoinits, gochecknoglobals) and interface-returning
  constructor (ireturn)

json_handler_test.go

@@ -1,22 +1,27 @@
-package simplelog
+package simplelog_test
 
 import (
 	"log/slog"
 	"testing"
 	"time"
+
+	"sneak.berlin/go/simplelog"
 )
 
 // TestJSONHandlerDeadlock verifies that JSONHandler.Handle does not deadlock
 // when the default slog handler routes log.Println back through slog.
 // On the unfixed code this test will hang (deadlock); with the fix it completes.
 func TestJSONHandlerDeadlock(t *testing.T) {
-	handler := NewJSONHandler()
+	t.Parallel()
+
+	handler := simplelog.NewJSONHandler()
 
 	// Set our handler as the default so log.Println routes through slog
 	logger := slog.New(handler)
 	slog.SetDefault(logger)
 
 	done := make(chan struct{})
+
 	go func() {
 		// This call deadlocks on unfixed code because Handle() calls
 		// log.Println() which re-enters slog → Handle() → log.Println() …