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.

.gitignore

@@ -0,0 +1,10 @@
+# Local environment
+.env
+
+# Go workspace (for local multi-module dev; see inventar ASR-0008)
+go.work
+go.work.sum
+
+# Build artifacts
+/bin/
+/dist/