refactor(go): restructure to /go/ subtree + audit COMPLIANT (Mantis #1227)

Bring go-cache to v0.9.0 audit-COMPLIANT verdict (every counter at 0).

Layout migration:
- Moved flat module into go/ subtree
- Added go.work at repo root with use ./go + ./external/{go,go-io}
- Added 2 git submodules (external/go, external/go-io) on dev branch
- go/go.mod declares `module dappco.re/go/cache`

Contract migration:
- Banned-import cleanup
- Result-discard propagation
- err-shape-funcs migrated to core.Result
- AX-7 triplet + Example coverage backfilled
- 0 gaming patterns

Verification: build / vet / test / audit.sh all clean.

Closes tasks.lthn.sh/view.php?id=1227

Co-authored-by: Codex <noreply@openai.com>

Author: Snider

.gitmodules

@@ -0,0 +1,8 @@
+[submodule "external/go"]
+	path = external/go
+	url = https://github.com/dappcore/go.git
+	branch = dev
+[submodule "external/go-io"]
+	path = external/go-io
+	url = https://github.com/dappcore/go-io.git
+	branch = dev

AGENTS.md

@@ -0,0 +1,11 @@
+# go-cache Agent Notes
+
+This repository follows the core/go v0.9 layout:
+
+- Go module source lives under `go/`.
+- Root `go.work` lists `./go` and local dependency checkouts under `external/`.
+- Do not edit `.core/` runtime configuration.
+- Do not edit source inside `external/`; only submodule pointers should change.
+
+Use `core.Result` for fallible package APIs and core/go wrappers for filesystem,
+JSON, process, string, and environment operations.

README.md

@@ -0,0 +1,20 @@
+# go-cache
+
+`dappco.re/go/cache` provides a storage-agnostic cache backed by
+`dappco.re/go/io` media. It stores JSON entries, binary sidecars, scoped caches,
+and request/response metadata for HTTP cache workflows.
+
+## Development
+
+The module lives in `go/`.
+
+```sh
+cd go
+GOWORK=off GOPROXY=direct GOSUMDB=off go test -count=1 -short ./...
+```
+
+The repository is v0.9.0 audit-driven; the release contract is:
+
+```sh
+bash /Users/snider/Code/core/go/tests/cli/v090-upgrade/audit.sh .
+```