feat: initial release — env decoder extracted from ai-fabric

Public API:
- Unmarshal(dst, opts...)
- UnmarshalPrefix(dst, prefix, opts...)
- AsMap() / AsMapPrefix(prefix)
- Options: WithTrim, WithWeaklyTyped, WithTagName, WithDecodeHook

Merges three divergent in-repo copies (ai-fabric/pkg/env,
markets-platform/TP-general-code/pkg/system, and the var/agents/issue-*
snapshots), adds error wrapping, prefix filter, and pluggable hooks.

15 pure-Go unit tests, no synthetic mocks, lint clean.

Extracted per inventar/docs/asr/ASR-0008-ai-fabric-audit.md.

doc.go

@@ -0,0 +1,28 @@
+// Package env reads process environment variables and decodes them into
+// Go structs using underscore-delimited paths.
+//
+// A variable named "SERVICE_HTTP_PORT" becomes the path Service.HTTP.Port
+// (case-insensitive, '_' is a path separator). Values are decoded via
+// github.com/mitchellh/mapstructure, so numeric, boolean and slice
+// conversions happen automatically.
+//
+//	var cfg struct {
+//		Service struct {
+//			HTTP struct {
+//				Port int
+//			}
+//			Key string
+//		}
+//	}
+//	if err := env.Unmarshal(&cfg); err != nil { ... }
+//
+// UnmarshalPrefix ignores variables that don't start with the given
+// prefix and strips it before building the path:
+//
+//	_ = os.Setenv("APP_DB_HOST", "localhost")
+//	_ = env.UnmarshalPrefix(&cfg, "APP_")
+//	// cfg.Db.Host == "localhost"
+//
+// String values are TrimSpace-trimmed by default; pass WithTrim(false) to
+// opt out.
+package env