Add README, LICENSE, Makefile, Dockerfile, and CI

Add a detailed README, WTFPL LICENSE, and build/CI tooling modeled on
the vaultik repo (Makefile, multi-stage digest-pinned Dockerfile,
.gitea/workflows/check.yml). Bump Go to 1.26.4 and pin golangci-lint
to v2.12.2. gofmt existing sources so the new fmt-check gate passes.

.gitea/workflows/check.yml

@@ -0,0 +1,14 @@
+name: check
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    steps:
+      # actions/checkout v4, 2024-09-16
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5
+      - name: Build and check
+        run: docker build .