chore: incorporate relevant agentbox skel files

Add relevant skeleton files from rcarmo/agentbox/skel for this repo:
- .github/workflows/ci.yml (Makefile checks + conditional Go tests)
- .github/instructions/{00-project-detection,docker-image,frontend-bun,go,python}.instructions.md

Instruction files were adapted where needed to match current workflow names
and repository layout (Go module under go/, Docker workflow filename).

Validated with make check and go test ./... .

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.github/instructions/go.instructions.md

@@ -0,0 +1,11 @@
+# Go project instructions
+
+Applies when: this repo has `go.mod`.
+
+## Makefile-first workflow
+- CI should run `make check` and Go tests (`cd go && go test ./...` in this repository).
+- Put `golangci-lint` and `gosec` wiring behind Make targets when introduced.
+
+## Conventions to implement
+- `make test` should run `go test ./...` (prefer `-race` and coverage in CI).
+- Avoid bespoke CI steps when a Make target can encode the same behavior.