feat: Add RelayKeys ZMK Integration Bridge (Phase 1)

[willwade]

This pull request implements Phase 1 of the "RelayKeys ZMK Integration" proposal, converting standard nRF52840 dongles into high-performance software-driven BLE HID bridges.

Key Changes

• ZMK Firmware Integration: Added firmware/zmk structure conforming to the ZMK module format, containing a standard Devicetree for a virtual/dummy matrix relaykeys_dongle shield that bypasses the hardware scanner.
• RPC HID Injector Module: Implemented a custom C module (relaykeys_handler.c) utilizing nanopb structural decoding to register a new ZMK RPC subsystem. This handler directly catches decoded payload buffers and ships them off using zmk_endpoints_send_report().
• Go Bridge Layer: Implemented the internal/zmkbridge package. It formats incoming RPC commands based on the SLIP-like literal framing specific to ZMK Studio (0xAB, 0xAC, 0xAD).
• Protobuf bindings: Defined relaykeys.proto to serialize and strictly type payload messages sent to the firmware, generating internal/zmk/proto Go mappings.
• Configurable Backend: The Go daemon configuration (internal/config/config.go & internal/rpc/server.go) now conditionally routes via FirmwareType. Legacy configurations fall back to the AT serial architecture, but those with the new ZMK dongle configuration bypass legacy constraints.

Next Steps (Phase 2)

The next task is to port mouse/consumer reports and administrative BLE management features to the host via new proto commands, as mapped out in the newly added AGENTS.md.

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PR created automatically by Jules for task 4908823840962447485 started by @willwade

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