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/docker-image.instructions.md

@@ -0,0 +1,12 @@
+# Docker image project instructions
+
+Applies when: this repo builds/publishes a Docker image.
+
+## CI/CD
+- Use `.github/workflows/docker.yml` (multi-arch by digest + manifest merge) as the baseline in this repository.
+- Tags `v*` are the release boundary.
+
+## Conventions
+- Keep Docker build args/env documented in README.
+- Prioritize building minimal images, taking advantage of caching and layers to remove build artifacts.
+- Prefer reproducible builds (minimize network fetches at runtime; pin versions when practical).