The push handler previously fell through to base64-decoding the
raw ciphertext whenever decryptDirect() returned null. For direct
(crypto_box) messages that produces garbage binary which surfaces
as garbled bytes in Claude's <channel> reminder. Limit the base64
fallback to legacy broadcast/channel messages (no senderPubkey),
and emit a clearer "⚠ message from <pubkey> failed to decrypt"
warning when direct decryption fails.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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