Re-vendor the shared files from sneak/prompts at dd4027b (closes #504)

The shared workflow, lint config, prettier settings and policies are the copies at `sneak/prompts` commit `dd4027b`. `.gitignore`, `.editorconfig` and `.dockerignore` are the shared copy followed by this repository's own entries. Linting is the image build's lint phase on golangci-lint v2.14.0, and tests run in their own test phase. Every scripted `docker build` passes `--no-cache`, so the CI fingerprint step and the superseded-run script are gone. The binary is built with `-trimpath -s -w`, and a build that has `.git` but no version fails. The development run keeps its databases outside the checkout.

Deviation: `.dockerignore` also leaves out SQLite databases at any depth.

Model: opus-5-5

script/css-check

@@ -1,14 +1,15 @@
 #!/bin/sh
 # script/css-check: fail when static/css/tailwind.css differs from what
 # script/css would generate (read-only). The comparison is the Dockerfile's
-# css-check stage, which the image build runs too.
+# css-check stage, which the image build runs too. --no-cache because a
+# cached check is a check that did not run.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    docker build --target css-check --output type=cacheonly .
+    docker build --no-cache --target css-check --output type=cacheonly .
 }
 
 main "$@"