cicd: Add a precommit hook to check formatting with gofmt. Add guide on how to add precommit hooks in the README

.githooks/pre-commit

@@ -0,0 +1,19 @@
+#!/usr/bin/env bash
+# Reject commits containing Go files that aren't gofmt-formatted.
+set -euo pipefail
+
+files="$(git diff --cached --name-only --diff-filter=ACM | grep '\.go$' || true)"
+
+if [ -z "$files" ]; then
+    exit 0
+fi
+
+unformatted="$(gofmt -l $files)"
+
+if [ -n "$unformatted" ]; then
+    echo "gofmt: the following files are not formatted:"
+    echo "$unformatted"
+    echo
+    echo "Run 'gofmt -w <file>...' and re-stage before committing."
+    exit 1
+fi

README.md

@@ -56,6 +56,12 @@ The intention for this project is to work as a template, where the developer sho
    go run ./cmd
    ```
 
+6. Enable the repo's git hooks (rejects commits with unformatted Go files):
+
+   ```bash
+   git config core.hooksPath .githooks
+   ```
+
 ## Migrations
 
 `goose.sh` loads `.env` and passes through to goose (`up`, `down`, `status`, ...).