chore: open-source release-prep (udev flag, SPDX headers, boolean style)

Three release cleanups, all behavior-preserving for my machines:

- Gated the Logitech BRIO udev rule behind INSTALL_DEVICE_UDEV_RULES (default yes, opt-out), so the device-specific rule is off for anyone without that hardware. Added the config read, validation, and a conf.example entry.
- Added a GPL-3.0-or-later SPDX-License-Identifier header after the shebang of all 24 shell scripts in the repo.
- Standardized boolean conditionals on the explicit [ "$var" = "true" ] form, replacing the bare `if $var` idiom. The STEPS function-dispatch is left alone, since it runs a function name rather than testing a boolean.

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