docs: CHANGELOG.md and CONTRIBUTING.md

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/),
+and this project adheres to [Semantic Versioning](https://semver.org/).
+
+## [0.1.0] - Unreleased
+
+### Added
+
+- Transparent LoRa radio over USB CDC-ACM (VID `1209`, PID `5741`)
+- COBS-framed postcard binary protocol (8 commands, 6 responses)
+- Radio configuration: frequency, bandwidth, spreading factor, coding rate, TX power
+- Continuous RX mode with RSSI/SNR per packet
+- Single-shot TX with optional per-packet config override
+- SSD1306 OLED display support with status dashboard
+  - Radio state, frequency, modulation parameters
+  - Packet counters (RX/TX)
+  - RSSI sparkline (1-minute history, TX shown as dotted bars)
+  - Combined splash/waiting screen on disconnect
+- Auto-detect USB host connect/disconnect via DTR
+- Config validation against SX1262 hardware limits
+- Board support: Heltec V3, Heltec V4, RAK WisBlock 4631
+- Example: MeshCore packet decoder with group channel decryption
+
+### Technical
+
+- Embassy async runtime (no threads, CPU sleeps when idle)
+- Zero-config: boots and waits for host commands
+- Protocol-agnostic: firmware has no opinion about what you transmit
+- Tasks never panic: all errors reported to host or logged
+- Board abstraction: one `.rs` file + one Cargo feature per board