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.

CHANGELOG.md

@@ -0,0 +1,32 @@
+# Changelog
+
+All notable changes to `go-env` are documented here.
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/)
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+## [0.1.0] - 2026-04-24
+
+Initial release. Extracted from `produktor.io/ai-fabric/pkg/env` per
+`inventar/docs/asr/ASR-0008-ai-fabric-audit.md`.
+
+### Added
+
+- `Unmarshal(dst, opts...)` — decode all process env vars into a Go struct.
+- `UnmarshalPrefix(dst, prefix, opts...)` — prefix-scoped variant that
+  strips the prefix before building the path.
+- `AsMap()` / `AsMapPrefix(prefix)` — escape hatch returning the nested
+  `map[string]any` built from `os.Environ()`.
+- Options: `WithTrim`, `WithWeaklyTyped`, `WithTagName`, `WithDecodeHook`.
+- TrimSpace on string values is enabled by default; disable with
+  `WithTrim(false)`.
+- Error wrapping with `%w` on both decoder configuration and decode
+  failures — matches `markets-platform/TP-general-code/pkg/system`
+  behaviour, improves on the original ai-fabric version.
+
+### Tests
+
+- 15 pure-Go unit tests. No mocks. `asMapFromEnviron` is tested with
+  synthetic `environ` slices (not a mock — a pure input). Follows the
+  eSlider `go-*` no-synthetic-mocks policy.