Add CI, repo CLAUDE.md, and finish Stage 0

- .gitea/workflows/ci.yaml: fmt/lint/test, same no-actions/checkout-and-manual-clone
  shape as terdut-server's ci.yaml, and the same reasoning for why (Node/ES2022
  incompatibility on the runner image). No chart/security jobs yet -- nothing for
  either to check until Stage 6 / real controller code exists.
- CLAUDE.md: Checks + Release sections, matching the sibling repos' convention from
  the workspace-level CLAUDE.md ("each repo has its own CLAUDE.md... read it before
  working in that repo"). Release is explicitly marked not-wired-yet rather than
  copying terdut-server's, since there's no chart to release against until Stage 6.
- ROADMAP.md: moved the .release.conf bullet out of Stage 0 (it names a HELM_CHART
  this repo doesn't have yet) -- it was already duplicated into Stage 6, which is
  where it actually belongs.

Stage 0 done: `make fmt lint test` verified green locally. Real open question the CI
workflow's comments flag rather than assume past: whether storage.googleapis.com
(envtest's binary source) is reachable from this Gitea runner's container network the
way proxy.golang.org is -- terdut-server's own ci.yaml notes get.helm.sh/github.com are
not. Only running the workflow for real will confirm; the comment names the fallback
(move the job out of `container:`, like terdut-server's chart job) if it isn't.

.gitea/workflows/ci.yaml

@@ -0,0 +1,68 @@
+name: CI
+
+# Same shape as terdut-server's ci.yaml: the release workflow gates a tag, which is
+# late, so this runs the same checks on the way in instead.
+#
+# push is scoped to main rather than all branches so a branch pushed as part of a pull
+# request is not checked twice.
+#
+# No actions/checkout, deliberately -- same reason as terdut-server: the runner image's
+# `nodejs` package predates ES2022 static initialiser blocks, so actions/checkout@v4
+# dies with `SyntaxError: Unexpected token '{'` before running. Cloning with git
+# directly avoids JS actions entirely. This repo is public, so the clone needs no
+# credential.
+#
+# `${{ }}` values are passed through `env:` and referenced as quoted shell variables --
+# a ref name is attacker-influenced by anyone who can push a branch or open a PR.
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+concurrency:
+  group: ci-${{ github.ref }}
+  cancel-in-progress: true
+
+env:
+  REPO_URL: https://git.ryuvia.com/niklas/terdut-operator.git
+
+jobs:
+  # `make fmt lint test` is exactly what a developer runs locally, so a green job here
+  # and a green working copy mean the same thing by construction. `test` also drives
+  # controller-gen/setup-envtest (via the Makefile's own `manifests generate ... test`
+  # chain), which needs storage.googleapis.com to fetch the envtest kube-apiserver/etcd
+  # binaries -- unconfirmed whether that host is reachable from this runner's dind
+  # bridge the way proxy.golang.org and git.ryuvia.com are (terdut-server's ci.yaml
+  # flags get.helm.sh and github.com as *not* reachable from here); if this job goes
+  # red on the fetch specifically rather than on a real test failure, move it out of
+  # `container:` the way the chart job in terdut-server's ci.yaml runs on the host
+  # instead, for the same "can't reach a fetch target from the dind bridge" reason.
+  test:
+    runs-on: ubuntu-latest
+    container:
+      image: golang:1.26.6-bookworm
+      volumes:
+        - go-mod-cache:/go/pkg/mod
+        - go-build-cache:/root/.cache/go-build
+        - gobin-cache:/go/bin
+
+    steps:
+      - name: Checkout
+        env:
+          REF_NAME: ${{ github.ref_name }}
+          HEAD_SHA: ${{ github.event.pull_request.head.sha }}
+        run: |
+          if [ -n "$HEAD_SHA" ]; then
+            # A pull_request ref_name is "<n>/merge", which is not a fetchable branch.
+            git clone "$REPO_URL" .
+            git checkout -q "$HEAD_SHA"
+          else
+            git clone --depth=1 --branch "$REF_NAME" "$REPO_URL" .
+          fi
+
+      - name: Format, lint and test
+        run: make fmt lint test
+
+  # No `chart` job yet -- there's no Helm chart until Stage 6 (ROADMAP.md). No
+  # `security` job yet either (govulncheck/gitleaks, as terdut-server has); add one
+  # alongside `test` once there's controller code worth scanning.