Add config file support and help to RPi scripts

- Add stegasoo.conf.example with all configurable options
- setup.sh: Add -h/--help, load config from /etc and ~/.config
- setup.sh: Support STEGASOO_BRANCH for non-main branches
- sanitize-for-image.sh: Add -h/--help with usage examples

Config files are loaded in order:
1. /etc/stegasoo.conf (system-wide)
2. ~/.config/stegasoo/stegasoo.conf (per-user)
3. Environment variables (highest priority)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

rpi/stegasoo.conf.example

@@ -0,0 +1,30 @@
+# Stegasoo Raspberry Pi Configuration
+# Copy this file to /etc/stegasoo.conf or ~/.config/stegasoo/stegasoo.conf
+#
+# You can also override these by exporting environment variables:
+#   export STEGASOO_INSTALL_DIR="/custom/path"
+#   ./setup.sh
+
+# Installation directory (default: /opt/stegasoo)
+#INSTALL_DIR="/opt/stegasoo"
+
+# Python version to install via pyenv (default: 3.12)
+#PYTHON_VERSION="3.12"
+
+# Git repository URL
+#STEGASOO_REPO="https://github.com/adlee-was-taken/stegasoo.git"
+
+# Git branch to checkout (default: main)
+#STEGASOO_BRANCH="main"
+
+# Web UI port (default: 5000)
+#STEGASOO_PORT="5000"
+
+# Enable HTTPS (default: false, configured via wizard)
+#STEGASOO_HTTPS_ENABLED="false"
+
+# Enable authentication (default: true)
+#STEGASOO_AUTH_ENABLED="true"
+
+# Channel key for private channels (default: none)
+#STEGASOO_CHANNEL_KEY=""