fix(buzz-cli): deliver documents as file cards instead of broken images

[kaminai0000]

Summary

buzz-cli is both stricter and less correct than the relay it talks to when
attaching a non-image file, so an agent cannot deliver any document to a
conversation. Two defects, neither visible to the sender.

Fixes #3083
Fixes #7324

1. Uploads were rejected client-side (#3083)

ALLOWED_MIMES listed only four image types plus video/mp4, so anything else
failed with unsupported file type: … before the request reached the relay.

The relay's /upload route already sniffs documents, archives, text and data
onto its generic-file path (buzz-media::validation, a deny-list that blocks
only active content and executables) and serves them with
Content-Disposition: attachment. Desktop users could attach a PDF; agents
could not.

2. Everything non-video was announced as an image (#7324)

The markdown embed came from a two-way branch: video/* got ![video](…) and
everything else got ![image](…). A document was therefore announced to the
client as an image and rendered as a permanently broken image placeholder —
while the send returned accepted: true, so the failure only ever appeared in
the recipient's client.

Generic files now use the same plain [filename](url) form Buzz Desktop emits
(imetaMediaMarkdown.ts::formatImetaMediaLine), which the renderer recognises
as a non-media blob and upgrades to a download card. The imeta tag carries
filename so the card has a label instead of the blob hash, and markdown
metacharacters in the label are escaped so a name like a].pdf cannot break
the link.

Testing

The embed choice is extracted into format_media_markdown so tests bind the
production branch rather than a copy of it. Reverting either fix turns the new
tests red — verified by re-introducing the ![image](…) catch-all, which fails
three of them.

• cargo test -p buzz-cli --lib — 469 passed
• cargo fmt --check -p buzz-cli, cargo clippy -p buzz-cli --all-targets — clean

Reproduced end to end against a self-hosted relay: a headless buzz-acp agent
generating an .xlsx could not attach it at all before this change, and
delivers it as a working download card after.

🤖 Generated with Claude Code

[github-actions]

🔐 Codex Security Review

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