ci: gate workflow + CI-image build/push, drop probe

- build-ci-image.yaml: builds .gitea/ci-image/Dockerfile via DinD and
  pushes git.lazyeval.net/oli/rdbms-playground-ci:latest (REGISTRY_*
  secrets); triggers on image-input changes + manual dispatch.
- ci.yaml: the gate — runs inside that image, clippy -D warnings +
  cargo test, on push/PR. fmt intentionally not gated (ADR-0049).

Removes ci-probe.yaml; it answered the runner questions (jobs run in
containers, host nix unreachable, custom container: works).

.gitea/workflows/ci.yaml

@@ -0,0 +1,24 @@
+# The CI gate. Runs inside the prebuilt nix toolchain image (built + pushed by
+# build-ci-image.yaml), so the pinned 1.95.0 toolchain is already warm — steps
+# just enter the flake devShell and run cargo.
+#
+# Gate = clippy + test. fmt is deliberately NOT gated yet (ADR-0049: the tree
+# isn't clean under stock rustfmt; revisit on main). The release job (static
+# binary for D2) and the platform matrix layer on later, step by step.
+name: ci
+on:
+  push:
+  pull_request:
+
+jobs:
+  gate:
+    runs-on: ci-public
+    # Public package → anonymous pull, no credentials needed.
+    container:
+      image: git.lazyeval.net/oli/rdbms-playground-ci:latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: clippy (warnings denied)
+        run: nix develop -c cargo clippy --all-targets -- -D warnings
+      - name: test
+        run: nix develop -c cargo test --no-fail-fast