Add sanitize script for distributable Pi images

- rpi/sanitize-for-image.sh: Removes personal data before imaging
  - Clears WiFi credentials
  - Removes SSH keys
  - Clears Stegasoo auth database
  - Removes logs, history, temp files
- Updated rpi/README.md with full image building workflow

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

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

rpi/README.md

@@ -97,3 +97,81 @@ rm -rf ~/stegasoo
 ## Pre-built Images
 
 Check [GitHub Releases](https://github.com/adlee-was-taken/stegasoo/releases) for pre-built SD card images.
+
+---
+
+## Building Your Own Image
+
+To create a distributable SD card image:
+
+### 1. Flash Fresh Raspberry Pi OS
+
+Use rpi-imager to flash Raspberry Pi OS (64-bit) to an SD card.
+
+In advanced settings, set:
+- Hostname: `stegasoo`
+- Enable SSH (password auth for initial setup)
+- Username/password (temporary, will work for any user)
+- Skip WiFi for distributable image
+
+### 2. Boot and Run Setup
+
+```bash
+# SSH into the Pi
+ssh pi@stegasoo.local
+
+# Run the setup script
+curl -sSL https://raw.githubusercontent.com/adlee-was-taken/stegasoo/main/rpi/setup.sh | bash
+```
+
+### 3. Test It Works
+
+```bash
+sudo systemctl start stegasoo
+curl -k https://localhost:5000  # Should return HTML
+```
+
+### 4. Sanitize for Distribution
+
+```bash
+# Download and run sanitize script
+curl -sSL https://raw.githubusercontent.com/adlee-was-taken/stegasoo/main/rpi/sanitize-for-image.sh | sudo bash
+```
+
+This removes:
+- WiFi credentials
+- SSH authorized keys
+- Bash history
+- Stegasoo auth database (users create their own admin)
+- Logs and temp files
+
+### 5. Create the Image
+
+After Pi shuts down, remove SD card and on another Linux machine:
+
+```bash
+# Find SD card device (BE CAREFUL - wrong device = data loss!)
+lsblk
+
+# Copy (replace sdX with your SD card)
+sudo dd if=/dev/sdX of=stegasoo-rpi-$(date +%Y%m%d).img bs=4M status=progress
+
+# Shrink the image (optional but recommended)
+wget https://raw.githubusercontent.com/Drewsif/PiShrink/master/pishrink.sh
+chmod +x pishrink.sh
+sudo ./pishrink.sh stegasoo-rpi-*.img
+
+# Compress
+xz -9 -T0 stegasoo-rpi-*.img
+```
+
+### 6. Distribute
+
+Upload the `.img.xz` file to GitHub Releases.
+
+Users flash with:
+```bash
+xzcat stegasoo-rpi-*.img.xz | sudo dd of=/dev/sdX bs=4M status=progress
+```
+
+Or use rpi-imager's "Use custom" option.