Re-vendor the canonical files from sneak/prompts at e3f79c4 (closes #221)

The vendored dotfiles, REPO_POLICIES.md and the standard scripts are
fetched from sneak/prompts commit e3f79c4, with pixa's own entries kept
after the standard part. The scripts pass a new CACHEBUST build argument
in place of --no-cache and every Dockerfile stage declares it first, so
every RUN step reruns while Go's build cache, mounted on the steps that
compile, is kept. The lint phase runs golangci-lint v2.14.0, the test
phase runs on the Debian Go image, and the build stage takes the version
as the policy gives it. The two new lint findings are fixed in the code.

Model: opus-5-5

.gitea/workflows/check.yml

@@ -1,15 +1,21 @@
 name: check
 on: [push]
+# Free the shared runner: a new push cancels only the same branch's older run.
+concurrency:
+    group: ${{ github.workflow }}-${{ github.ref }}
+    cancel-in-progress: true
 jobs:
     check:
         runs-on: ubuntu-latest
+        # Free the shared runner from a hung build.
+        timeout-minutes: 20
         steps:
             # actions/checkout v4.2.2, 2026-02-22
             - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683
-              # The default clone is shallow and has no tags, so the
-              # version the build takes from `git describe` would be a
-              # bare commit; this fetches the whole history with its tags.
+              # script/cibuild needs no token, so none is left in .git/config.
               with:
+                  persist-credentials: false
+                  # All history and tags, so git describe finds the version tag.
                   fetch-depth: 0
             - run: script/cibuild
             - run: script/docker-smoke