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.

README.md

@@ -0,0 +1,97 @@
+# go-env
+
+[![Go Reference](https://pkg.go.dev/badge/github.com/eslider/go-env.svg)](https://pkg.go.dev/github.com/eslider/go-env)
+
+Tiny, zero-ceremony library for decoding process environment variables into
+Go structs.
+
+- Uses `_` as a path separator: `SERVICE_HTTP_PORT` → `Service.HTTP.Port`.
+- Weakly-typed by default (`"1"` → `int`, `"true"` → `bool`).
+- Optional prefix filter with automatic stripping.
+- Pluggable decode hooks via [`mitchellh/mapstructure`][mapstructure].
+
+## Install
+
+```sh
+go get github.com/eslider/go-env
+```
+
+## Quick start
+
+```go
+package main
+
+import (
+	"fmt"
+
+	"github.com/eslider/go-env"
+)
+
+type Config struct {
+	Service struct {
+		HTTP struct {
+			Port int
+		}
+		Key string
+	}
+}
+
+func main() {
+	var cfg Config
+	if err := env.Unmarshal(&cfg); err != nil {
+		panic(err)
+	}
+	fmt.Printf("%+v\n", cfg)
+}
+```
+
+With a prefix:
+
+```go
+// Only looks at APP_* variables; strips the APP_ prefix before decoding.
+_ = env.UnmarshalPrefix(&cfg, "APP_")
+```
+
+## API
+
+| Function | Purpose |
+|---|---|
+| `Unmarshal(dst, opts...)` | Decode all env vars into `dst`. |
+| `UnmarshalPrefix(dst, prefix, opts...)` | Same, but only vars starting with `prefix`. |
+| `AsMap()` / `AsMapPrefix(prefix)` | Return the nested `map[string]any` used by the decoder (debugging, custom decoders). |
+
+### Options
+
+| Option | Default | Purpose |
+|---|---|---|
+| `WithTrim(bool)` | `true` | TrimSpace every string value. |
+| `WithWeaklyTyped(bool)` | `true` | `mapstructure`'s weakly-typed coercion. |
+| `WithTagName(string)` | `"mapstructure"` | Struct tag name for field overrides. |
+| `WithDecodeHook(h)` | — | Append a `mapstructure.DecodeHookFunc` to the chain. |
+
+## Semantics
+
+- **Path collisions**: first write wins. If both `FOO=1` and `FOO=2` exist
+  in the environ, only `FOO=1` is kept. This matches the original
+  `ai-fabric/pkg/env` behaviour.
+- **Case-insensitive keys**: all path components are lower-cased; struct
+  fields are matched via `mapstructure` which is also case-insensitive.
+- **`_`-only delimiter**: there's no escape — if a variable legitimately
+  contains `_` inside a "leaf" name, you must restructure your struct to
+  match the nested layout.
+
+## Status
+
+Extracted from `produktor.io/ai-fabric` as part of the eSlider `go-*`
+library standard (ASR-0008). Merges the best parts of three previously
+divergent copies:
+
+- `produktor.io/ai-fabric/pkg/env`
+- `markets-platform/TP-general-code/pkg/system/env.go`
+- the various `pkg/system/env.go` snapshots inside `var/agents/issue-*/`
+
+## License
+
+MIT © Andriy Oblivantsev
+
+[mapstructure]: https://github.com/mitchellh/mapstructure