Add reflective struct-tag config loader

Implements conf.Load to populate tagged structs from a chain of Sources
(env, .env, YAML/JSON/TOML, custom). Supports default values, slice
separators, nested structs, pointer fields, and a Validator hook.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

source.go

@@ -0,0 +1,34 @@
+package conf
+
+import "os"
+
+// Source returns a string value for a key, or false if the key is not present.
+type Source interface {
+	Lookup(key string) (string, bool)
+}
+
+type envSource struct{}
+
+func (envSource) Lookup(key string) (string, bool) {
+	v, ok := os.LookupEnv(key)
+	if !ok || v == "" {
+		return "", false
+	}
+	return v, true
+}
+
+// EnvSource reads from the process environment. Empty values are treated as absent.
+func EnvSource() Source { return envSource{} }
+
+type mapSource map[string]string
+
+func (m mapSource) Lookup(key string) (string, bool) {
+	v, ok := m[key]
+	if !ok || v == "" {
+		return "", false
+	}
+	return v, true
+}
+
+// MapSource wraps a key/value map as a Source. The map is not copied.
+func MapSource(m map[string]string) Source { return mapSource(m) }