Add a workstation deploy script

The Gitea runner is not up yet, so deploys need to be possible by hand.
scripts/deploy.sh mirrors the workflow: same hugo build, same rsync
invocation, same DEPLOY_PATH guards.

It previews every change and reports the number of server-side deletions
before asking to continue, since rsync runs with --delete. Credentials
live in the gitignored scripts/deploy.env.

Two Windows-specific guards, both hit in practice: it refuses an SSH key
under /mnt (WSL cannot hold 0600 there, so ssh rejects it) and it fails
with a clear message when run from Git Bash, which has no rsync.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

scripts/deploy.env.example

@@ -0,0 +1,10 @@
+# Copy to scripts/deploy.env (gitignored) and fill in.
+# Must match the values used by the Gitea Actions secrets.
+
+DEPLOY_HOST=server.example.de
+DEPLOY_USER=www-deploy
+DEPLOY_PATH=/var/www/mcg
+
+# Private half of the deploy key. Keep it inside the WSL filesystem —
+# a key under /mnt/c or /mnt/d cannot hold 0600 and ssh will reject it.
+SSH_KEY=$HOME/.ssh/mcg-deploy