Re-vendor the canonical files from sneak/prompts at dd4027b (closes #213)

Linting and testing become the lint and test phases of the Dockerfile,
and the build stage depends on both. Dockerfile.lint, CHECK_EPOCH and
the tests that checked them are removed. Every docker build in script/
passes --no-cache, and script/cibuild runs script/bootstrap first. The
image takes its version from the VERSION build arg or git describe, and
still stamps the commit and its date from .git. The golangci-lint
v2.14.0 findings are fixed in the code. The rules in CLAUDE.md move into
AGENTS.md. IsDevVersion now counts "unknown", the version script/docker
stamps outside a git checkout.

Model: opus-5-5

.gitea/workflows/check.yml

@@ -1,14 +1,9 @@
 name: check
-on:
-  push:
-    branches: [main, next]
-  pull_request:
-    branches: [main, next]
+on: [push]
 jobs:
-  check:
-    runs-on: ubuntu-latest
-    steps:
-      # actions/checkout v4, 2024-09-16
-      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5
-      - name: Build and check
-        run: script/cibuild
+    check:
+        runs-on: ubuntu-latest
+        steps:
+            # actions/checkout v4.2.2, 2026-02-22
+            - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683
+            - run: script/cibuild