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 Every scripted `docker build` passes `--no-cache`, so the CI fingerprint step and the superseded-run script are gone.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/fmt

@@ -1,7 +1,8 @@
 #!/bin/sh
 # script/fmt: format all files (writes): the Go code with gofmt and
 # goimports, the Markdown with prettier. prettier is never installed
-# locally: it runs in docker, in the Dockerfile's Markdown stages.
+# locally: it runs in docker, in the Dockerfile's Markdown stages, built
+# with --no-cache like every docker build in script/.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
@@ -12,7 +13,8 @@ main() {
     if command -v goimports >/dev/null 2>&1; then
         goimports -w .
     fi
-    docker build --target markdown-output --output type=local,dest=. .
+    docker build --no-cache \
+        --target markdown-output --output type=local,dest=. .
 }
 
 main "$@"