fix(cli): support native document attachments

[chigreen]

Summary

• allow buzz messages send --file to upload generic files accepted by the relay instead of enforcing a media-only client allowlist
• enforce the relay-compatible 100 MB generic-file size ceiling while preserving the existing image/video ceilings
• emit generic files as file links rather than broken image markdown
• include the original basename in the imeta filename field so Buzz renders and downloads the native file with the correct name

Root cause

Hermes could hand a document to buzz-cli, but buzz-cli rejected Office/PDF/archive MIME types before calling the relay. Hermes then surfaced the platform fallback marker rather than a native attachment.

Verification

• cargo test -p buzz-cli — 369 passed
• cargo clippy -p buzz-cli -- -D warnings — passed
• cargo build -p buzz-cli --release — passed
• live XLSX upload to Buzz succeeded
• event readback contained native imeta metadata with the original filename and XLSX MIME
• downloaded Blossom blob SHA-256 matched the source workbook

Live verification event: 23a6017fdce4923d8a45bb6c90d9d91fccba83ba6c22e7be8c6e226a5612e8aa.