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.

.golangci.yml

@@ -0,0 +1,23 @@
+run:
+  timeout: 5m
+  tests: true
+
+linters:
+  disable-all: true
+  enable:
+    - errcheck
+    - gofmt
+    - goimports
+    - govet
+    - ineffassign
+    - revive
+    - staticcheck
+    - unused
+
+linters-settings:
+  revive:
+    rules:
+      - name: var-naming
+      - name: exported
+      - name: package-comments
+      - name: indent-error-flow