Add CI policy, strengthen hash-pinning rule, add Gitea Actions workflow

- All Dockerfiles must run make check as a build step
- Every repo needs a Gitea Actions workflow running docker build on push
- Greatly strengthen the hash-pinning rule: explicitly list all reference
  types, ban curl|bash installs, mark as most important rule in document
- Add model .gitea/workflows/check.yml pinned by commit hash

.gitea/workflows/check.yml

@@ -0,0 +1,9 @@
+name: check
+on: [push]
+jobs:
+    check:
+        runs-on: ubuntu-latest
+        steps:
+            # actions/checkout v4.2.2, 2026-02-22
+            - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683
+            - run: docker build .