fix: reject empty message content

[jaezinpark]

Summary

• reject messages whose trimmed content and attachment list are both empty
• fail before network publication with a user error that points to missing stdin
• preserve normal text messages and attachment-only posts

Motivation

Two agent callbacks invoked messages send with --content - but delivered EOF. The CLI accepted and published blank events, so accepted:true masked the caller failure.

Verification

• cargo fmt --all -- --check
• cargo test -p buzz-cli (366 passed)
• cargo clippy -p buzz-cli -- -D warnings
• empty stdin smoke: exit 1, stdout 0 bytes, retryable false
• whitespace-only stdin smoke: exit 1, stdout 0 bytes, retryable false