diskOS installer: initial public beta

Flashes the diskOS custom UI onto the FiiO Snowsky Disc over Ingenic mask-ROM USB,
building the image from your own stock firmware. Runs from source via install.sh.

CONTRIBUTING.md

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+# Contributing to diskOS
+
+Thanks for wanting to help. diskOS touches real hardware in a way that can brick a device, so a few
+rules keep contributions safe and legally clean.
+
+## Ground rules
+
+1. **Hardware-affecting changes must be tested on real hardware.** Anything that changes the image
+   builder, the boot hook, the NAND writer, the SPL, or device targeting has to be flashed and
+   booted on an actual Snowsky Disc before it is declared working. Say which firmware version and
+   which DRAM/NAND variant you tested on.
+2. **Never weaken the fail-closed contract.** The first-boot hook must always fall back to the stock
+   UI when it cannot verify the UI against the baked manifest. The flasher must fail closed rather
+   than commit an unverified or wrong-device write.
+3. **Add an error code for every new failure mode** (`E1xx` preflight, `E2xx` build, `E3xx` flash,
+   `F1xx` device-writer) and document it in `README.md`.
+4. **Keep the honest-limitations tone.** If something is blocked by hardware or a toolchain gap, say
+   so plainly. Do not imply a feature works when it has not been verified.
+
+## Licensing and provenance
+
+- Original diskOS files are MIT. By contributing you agree your contribution is under the same
+  license. Please sign off your commits (`git commit -s`, Developer Certificate of Origin).
+- **Do not upload FiiO firmware, stock rootfs images, or generated `diskos_*.bin` images** to issues
+  or PRs - they contain FiiO's software. The installer builds images locally from a user's own
+  firmware; that is fine, redistributing them is not.
+- If you touch how a GPL/LGPL component is bundled (usbboot, squashfs-tools, the SPL, libusb),
+  keep its corresponding source and `NOTICE.md` in sync.
+- Do not commit device-identifying data (serials, MAC addresses) or secrets.
+
+## Filing issues
+
+Include: firmware version + hash, DRAM/NAND variant if known, host OS, the exact error code, the
+bad-block count if the flasher printed one, and redacted logs. **Do not** post serial numbers, MAC
+addresses, firmware zips, stock rootfs, or generated images.
+
+## Development
+
+See [`agents/AGENTS.md`](agents/AGENTS.md) for a project brief (the hardware facts that are easy to
+get wrong, the repo layout, and the device-safety rules) - useful whether you work by hand or with
+an AI coding agent. Build the installer with `bash build/build.sh`.