refactor(go): restructure to /go/ subtree (Mantis #1236)

- Move all .go sources from repo root into /go/ subtree (canonical layout)
- Add go.work at repo root + external/go submodule
- Repo was already audit-COMPLIANT in flat layout — this commit lands the
  canonical repo-shape (Layer 1 of the unified brief).
- go mod tidy regenerated go.sum to match upstream proxy

Verification:
- GOWORK=off go build ./...    clean
- GOWORK=off go vet ./...      clean
- GOWORK=off go test -count=1 -short ./...  clean
- audit.sh verdict: COMPLIANT (every counter at 0)

Closes tasks.lthn.sh/view.php?id=1236

Author: Snider

.gitmodules

@@ -0,0 +1,4 @@
+[submodule "external/go"]
+	path = external/go
+	url = https://github.com/dappcore/go.git
+	branch = dev

external/go

@@ -1,3 +1,10 @@
 go 1.26.0
 
-use .
+// Workspace mode for development: pulls local sources from external/ submodules.
+//
+// CI: GOWORK=off uses go/go.mod tags for reproducible resolution.
+
+use (
+	./go
+	./external/go
+)

go.sum

@@ -0,0 +1,2 @@
+dappco.re/go v0.9.0 h1:4ruZRNqKDDva8o6g65tYggjGVe42E6/lMZfVKXtr3p0=
+dappco.re/go v0.9.0/go.mod h1:xapr7fLK4/9Pu2iSCr4qZuIuatmtx1j56zS/oPDbGyQ=