Re-vendor the canonical files from sneak/prompts at dd4027b (closes #112)

The shared files are fetched from sneak/prompts dd4027b, with this
repository's own entries after the shared content in .gitignore,
.editorconfig and .dockerignore.

Lint and tests are Dockerfile phases (golangci-lint v2.14.0, Debian Go
1.24.13) that the build stage depends on, and the Makefile targets call
the script/ entrypoints. make fmt also formats Markdown with prettier.

Fixes for the new lint findings: the auth cookie is always Secure, an
IRC connection's relay goroutine stops when the connection closes, and
repeated strings are constants.

Model: opus-5-5

script/precommit

@@ -0,0 +1,23 @@
+#!/bin/sh
+# script/precommit: run by the git pre-commit hook; fails the commit if
+# checks fail. Our own extension to scripts-to-rule-them-all.
+#
+# This repo's addition: go mod tidy runs first, failing the commit if it
+# changes go.mod or go.sum.
+set -eu
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
+ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
+
+main() {
+    cd "$ROOT"
+    go mod tidy
+    git diff --exit-code -- go.mod go.sum || {
+        echo "precommit: go mod tidy changed go.mod or go.sum;" \
+            "stage the changes and retry" >&2
+        exit 1
+    }
+    "$SCRIPT_DIR/check"
+}
+
+main "$@"