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.

AGENTS.md

@@ -0,0 +1,50 @@
+# AGENTS.md — `go-env`
+
+This module is part of the eSlider `go-*` library standard (inventar ASR-0008).
+
+## Purpose
+
+Decode `os.Environ()` into Go structs. Extracted from
+`produktor.io/ai-fabric/pkg/env` on 2026-04-24.
+
+## Public API surface
+
+- `env.Unmarshal(dst any, opts ...Option) error`
+- `env.UnmarshalPrefix(dst any, prefix string, opts ...Option) error`
+- `env.AsMap() map[string]any`
+- `env.AsMapPrefix(prefix string) map[string]any`
+- Options: `WithTrim`, `WithWeaklyTyped`, `WithTagName`, `WithDecodeHook`
+
+Breaking changes require a new major version tag (SemVer). Internal helpers
+(`asMapFromEnviron`, `trimStringHook`, `insertPath`) are unexported and
+may change without notice.
+
+## Testing policy
+
+Follows the eSlider "no synthetic mocks" policy:
+
+- **Unit tests** (`env_test.go`): pure inputs only. We pass synthetic
+  `environ` slices to `asMapFromEnviron` — that is not a mock of anything
+  external; it's the normal way to test a pure function. Use `t.Setenv`
+  for `Unmarshal*` happy-path tests since Go's `os.Environ` lookup is
+  well-defined local behaviour, not a vendor protocol.
+- **No `httptest`** — this library has no HTTP surface.
+- **No integration-test build tag** — everything is in-process.
+
+## Checklist before release
+
+```sh
+cd go-env
+go mod tidy
+go vet ./...
+go test -race -count=1 ./...
+golangci-lint run --timeout 5m   # same preset as go-onlyoffice
+```
+
+Bump `CHANGELOG.md`, tag `vX.Y.Z`, push.
+
+## Related
+
+- `inventar/docs/asr/ASR-0008.md` — Go library module conventions
+- `inventar/docs/asr/ASR-0008-ai-fabric-audit.md` — why this module exists
+- `go-onlyoffice/AGENTS.md` — reference for the eSlider library template