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>

README.md

@@ -60,6 +60,10 @@ type MyConfig struct {
 
 cfg, err := config.Load[MyConfig]("my-service.toml", "MYSERVICE")
 
+// Timeout fields use config.Duration (go-toml v2 doesn't decode
+// strings to time.Duration natively). Access via .Duration:
+readTimeout := cfg.Server.ReadTimeout.Duration  // time.Duration
+
 // Open database with standard pragmas and run migrations.
 database, err := db.Open(cfg.Database.Path)
 migrations := []db.Migration{