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 Every scripted `docker build` passes `--no-cache`, so the CI fingerprint step and the superseded-run script are gone.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

@@ -3,8 +3,7 @@
 # Docker: Dockerfile.browser builds the test and runs it in a digest-pinned
 # headless browser image, so the host needs no browser.
 #
-# --no-cache-filter=browser runs the test again even when nothing changed;
-# it must name the stage in Dockerfile.browser that runs it.
+# --no-cache because a cached test layer is a test that did not run.
 # --output=type=cacheonly leaves no image behind to clean up.
 set -eu
 
@@ -14,7 +13,7 @@ main() {
     cd "$ROOT"
     docker build \
         -f Dockerfile.browser \
-        --no-cache-filter=browser \
+        --no-cache \
         --progress=plain \
         --output=type=cacheonly \
         .