feat: publish development source without USB storage

[openbrt]

Summary

• publish the current development firmware, official Agent changes, and sanitized Kindle reference source
• remove the USB MSC product disk, FAT partition, TinyUSB MSC dependency, release defaults, and U-disk user copy
• retain USB Serial/JTAG configuration and serial flashing support
• tighten the public release boundary and exclude runtime/generated data

Verification

• python3 scripts/check_firmware_regressions.py
• ESP-IDF 5.5.2 public build: Waveshare ESP32-S3-RLCD-4.2
• ESP-IDF 5.5.2 public build: Zectrix Note 4
• WCB_MODEL_STUB=1 .venv/bin/python -m tests.e2e
• WCB_MODEL_STUB=1 .venv/bin/python proofs/run_display_shadow_proof.py
• Kindle Node/Shell syntax checks and reference-server smoke test
• staged secret/private-network/banned-path audit

No firmware Release or installer asset is created by this source-only merge.