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/config.go

@@ -5,6 +5,24 @@
 // the standard sections (Server, Database, MCIAS, Log). Use [Load] to
 // parse a TOML file, apply environment overrides, set defaults, and
 // validate required fields.
+//
+// # Duration fields
+//
+// Timeout fields in [ServerConfig] use the [Duration] type rather than
+// [time.Duration] because go-toml v2 does not natively decode strings
+// (e.g., "30s") into time.Duration. Access the underlying value via
+// the embedded field:
+//
+//	cfg.Server.ReadTimeout.Duration  // time.Duration
+//
+// In TOML files, durations are written as Go duration strings:
+//
+//	read_timeout = "30s"
+//	idle_timeout = "2m"
+//
+// Environment variable overrides also use this format:
+//
+//	MCR_SERVER_READ_TIMEOUT=30s
 package config
 
 import (