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/frontend-bun.instructions.md

@@ -0,0 +1,10 @@
+# Frontend (Bun/Node) instructions
+
+Applies when: this repo has `package.json` and a JS/TS build step.
+
+## Makefile-first workflow
+- Expose frontend workflows via Make targets (`typecheck`, `build`, `build-fast`, `bundle-watch`, `bundle-clean`).
+- Keep CI invoking those targets (or `make check` if it wraps them).
+
+## Conventions
+- Bun is preferred for install/build when available.