docs: add docker, systemd, and git examples

sample_pet/README

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 Pets configuration examples.
+
+This directory contains sample pet files demonstrating various use cases:
+
+cron/
+  certbot    - Let's Encrypt certificate renewal cron job
+  mdadm      - RAID monitoring cron job
+
+docker/
+  docker-compose.yml - Deploy a docker-compose stack (nginx + redis)
+
+git/
+  gitconfig  - System-wide git configuration with sensible defaults
+
+ssh/
+  sshd_config    - SSH daemon configuration with key-only auth
+  user_ssh_config - User SSH client configuration
+
+ssmtp/
+  ssmtp.conf  - Simple SMTP relay configuration
+  revaliases  - Reverse aliases for outgoing mail
+
+sudo/
+  sudoers     - Sudo configuration
+  sudo_group  - Sudo group membership
+
+systemd/
+  backup.service - Systemd service unit for scheduled backups
+  backup.timer   - Systemd timer for daily backup execution
+
+vimrc - Vim editor configuration

sample_pet/docker/docker-compose.yml

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+# pets: destfile=/opt/myapp/docker-compose.yml, owner=root, group=docker, mode=0640
+# pets: package=docker.io
+# pets: package=docker-compose
+# pets: post=/usr/bin/docker-compose -f /opt/myapp/docker-compose.yml up -d
+#
+# Example: Deploy a simple web application with nginx and redis
+# Run 'pets' to install docker packages and deploy the compose stack
+
+version: '3.8'
+
+services:
+  web:
+    image: nginx:alpine
+    ports:
+      - "80:80"
+    volumes:
+      - ./html:/usr/share/nginx/html:ro
+    depends_on:
+      - cache
+    restart: unless-stopped
+
+  cache:
+    image: redis:alpine
+    volumes:
+      - redis_data:/data
+    restart: unless-stopped
+
+volumes:
+  redis_data:

sample_pet/git/gitconfig

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+# pets: destfile=/etc/gitconfig, owner=root, group=root, mode=0644
+# pets: package=git
+#
+# Example: System-wide git configuration
+# Sets sensible defaults for all users on the system
+
+[init]
+    defaultBranch = main
+
+[core]
+    autocrlf = input
+    editor = vim
+
+[pull]
+    rebase = true
+
+[push]
+    default = current
+    autoSetupRemote = true
+
+[alias]
+    st = status
+    co = checkout
+    br = branch
+    ci = commit
+    lg = log --oneline --graph --decorate

sample_pet/systemd/backup.service

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+# pets: destfile=/etc/systemd/system/backup.service, owner=root, group=root, mode=0644
+# pets: post=/bin/systemctl daemon-reload
+#
+# Example: A systemd service unit for scheduled backups
+# Use with backup.timer to run backups on a schedule
+
+[Unit]
+Description=System backup service
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=oneshot
+ExecStart=/usr/local/bin/backup.sh
+StandardOutput=journal
+StandardError=journal
+
+[Install]
+WantedBy=multi-user.target

sample_pet/systemd/backup.timer

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+# pets: destfile=/etc/systemd/system/backup.timer, owner=root, group=root, mode=0644
+# pets: post=/bin/systemctl daemon-reload
+# pets: post=/bin/systemctl enable --now backup.timer
+#
+# Example: A systemd timer to trigger daily backups at 3am
+
+[Unit]
+Description=Run backup daily
+
+[Timer]
+OnCalendar=*-*-* 03:00:00
+Persistent=true
+RandomizedDelaySec=1800
+
+[Install]
+WantedBy=timers.target