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)

console_handler.go

@@ -4,26 +4,38 @@ import (
 	"context"
 	"fmt"
 	"log/slog"
+	"os"
 	"runtime"
 	"time"
 
 	"github.com/fatih/color"
 )
 
+// callerSkipFrames is the number of stack frames between runtime.Caller
+// and the slog call site that produced the record.
+const callerSkipFrames = 4
+
+// ConsoleHandler writes human-readable, colored log lines to stdout.
 type ConsoleHandler struct{}
 
+// NewConsoleHandler returns a new ConsoleHandler.
 func NewConsoleHandler() *ConsoleHandler {
 	return &ConsoleHandler{}
 }
 
+// Handle writes the record to stdout as a colored, timestamped line
+// including the caller file and line.
 func (c *ConsoleHandler) Handle(
-	ctx context.Context,
+	_ context.Context,
 	record slog.Record,
 ) error {
 	timestamp := time.Now().UTC().Format("2006-01-02T15:04:05.000Z07:00")
-	var colorFunc func(format string, a ...interface{}) string
+
+	var colorFunc func(format string, a ...any) string
 
 	switch record.Level {
+	case slog.LevelDebug:
+		colorFunc = color.New(color.FgWhite).SprintfFunc()
 	case slog.LevelInfo:
 		colorFunc = color.New(color.FgBlue).SprintfFunc()
 	case slog.LevelWarn:
@@ -35,12 +47,14 @@ func (c *ConsoleHandler) Handle(
 	}
 
 	// Get the caller information
-	_, file, line, ok := runtime.Caller(4)
+	_, file, line, ok := runtime.Caller(callerSkipFrames)
 	if !ok {
 		file = "???"
 		line = 0
 	}
-	fmt.Println(
+
+	_, _ = fmt.Fprintln(
+		os.Stdout,
 		colorFunc(
 			"%s [%s] %s:%d: %s",
 			timestamp,
@@ -50,20 +64,25 @@ func (c *ConsoleHandler) Handle(
 			record.Message,
 		),
 	)
+
 	return nil
 }
 
+// Enabled reports whether the handler processes records at the given
+// level; it always returns true.
 func (c *ConsoleHandler) Enabled(
-	ctx context.Context,
-	level slog.Level,
+	_ context.Context,
+	_ slog.Level,
 ) bool {
 	return true
 }
 
-func (c *ConsoleHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
+// WithAttrs returns the handler unchanged; attributes are not rendered.
+func (c *ConsoleHandler) WithAttrs(_ []slog.Attr) slog.Handler {
 	return c
 }
 
-func (c *ConsoleHandler) WithGroup(name string) slog.Handler {
+// WithGroup returns the handler unchanged; groups are not rendered.
+func (c *ConsoleHandler) WithGroup(_ string) slog.Handler {
 	return c
 }