Fix Pi setup: use /opt/stegasoo, flexible Python version

- Change .python-version from 3.12.0 to 3.12 (matches any 3.12.x)
- Update docs to use /opt/stegasoo instead of ~/stegasoo
- Add pre-setup steps: chown /opt, install git
- Renumber BUILD_IMAGE.md steps (now 9 steps)

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

.python-version

@@ -1 +1 @@
-3.12.0
+3.12