Document Duration wrapper type usage

- config package doc: explain Duration fields, TOML format, env vars
- duration.go: expanded godoc with access pattern examples
- README: show .Duration access in quick start

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

config/duration.go

@@ -5,9 +5,18 @@ import (
 	"time"
 )
 
-// Duration is a time.Duration that can be unmarshalled from a TOML string
+// Duration is a [time.Duration] that can be unmarshalled from a TOML string
 // (e.g., "30s", "5m"). go-toml v2 does not natively decode strings into
-// time.Duration, so this wrapper implements encoding.TextUnmarshaler.
+// time.Duration, so this wrapper implements [encoding.TextUnmarshaler].
+//
+// Access the underlying time.Duration via the embedded field:
+//
+//	cfg.Server.ReadTimeout.Duration  // time.Duration value
+//
+// Duration values work directly with time functions that accept
+// time.Duration because of the embedding:
+//
+//	time.After(cfg.Server.ReadTimeout.Duration)
 type Duration struct {
 	time.Duration
 }