chore: initial hub repo structure

docs/superpowers/specs/2026-05-26-remove-nix-docker-only-design.md

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+# Hapus Nix, Docker-Only
+
+**Goal:** Remove all Nix configuration and adapt CI/dev to Docker-only.
+
+**Scope:**
+- Delete: `flake.nix`, `flake.lock`, `nix/`, `.envrc`, `.direnv/`
+- Modify: `.github/workflows/docker-build-push.yml` — remove Nix steps, move `rust-api` to Dockerfile build
+- Modify: `.vscode/settings.json` — remove `nixEnvSelector`
+- Modify: `README.md` — replace `nix build` with `docker build`
+
+**CI Changes:**
+- `rust-api` uses `infra/docker/rust.Dockerfile` (already exists)
+- Remove `cachix/install-nix-action`, `cachix/cachix-action`, `nix build` step
+- Remove `flake.nix`, `flake.lock`, `nix/**` from path triggers
+- Update path filters: `nix/apps/*.nix` → `infra/docker/*.Dockerfile`
+- Remove `repository_dispatch` Nix `--override-input` logic
+- Add `rust-api` to Dockerfile case + push branch
+
+**Dev Workflow:**
+- Rust toolchain via `rustup`/`rust-toolchain.toml` in `apps/rust/`
+- Dev via `docker compose` instead of `process-compose`