[BUG] Require clean USB installs and use OTA for updates

[EdwardChamberlain]

Summary

• Reject USB deployments unless --clean is explicit, before any board writes.
• Explain that clean USB installation erases settings and application state; direct application updates should use OTA.
• Show the same clean-install warning in the desktop setup assistant.

Tests: python3 -m pytest tests/test_push_micro.py -q (5 passed); python3 -m py_compile scripts/push_micro.py src/bambutton/gui.py; git diff --check.

Closes #46

[EdwardChamberlain]

Review of 07177bb35e56638ec2b559ad23c2884cf9dd3bfb: no blocking findings.

The CLI rejects a USB installation without --clean before invoking mpremote, and the production desktop path erases firmware/files before the bootstrap handoff. The clean-install warning is visible in the GUI and README. The staged bootloader handoff is preserved.

Verification: 30 USB/GUI regression tests passed locally with a FreeSimpleGUI import shim; the current CI run is green. Complete the consistent OTA/clean-reinstall documentation through PR #53. This review did not repeat physical board testing.