feat(cli): let messages send attach a link-preview snapshot tag

**Motivation**

An agent posts links into a channel on a self-hosted relay. The Buzz desktop client renders those links bare, while the same links posted by a human from the desktop app render as rich cards.

The card comes from a tag the *sender* embeds, not from anything the recipient does - `desktop/src/shared/ui/markdown/useMessageLinkPreviews.ts` states the privacy model explicitly ("recipients never contact external sites"), and `desktop/src/shared/lib/linkPreviewSnapshot.ts` parses it:

```
["link-preview", "snapshot", "1", , , <site>, <description>,
<image url>, <image sha256>, <favicon url>, <favicon sha256>]
```

The desktop app writes it through `desktop/src-tauri/src/link_preview_tags.rs`, whose validation matches the reader's. An image must be relay-hosted at `/media/<sha256>.<jpg|png|gif|webp>` with a matching hash, which `buzz upload file` already produces - so the only missing piece is attaching the tag.

The gap: `crates/buzz-cli/src/commands/messages.rs` builds an event's tags entirely from `SendMessageParams` - channel, content, kind, reply-to, broadcast, files, mentions. There is no way to pass an arbitrary tag, so a CLI or SDK client cannot produce a card that the app produces routinely. Non-desktop senders (agents, bots, CI, mobile tooling) are second-class in the timeline as a result.

**Proposed solution**

`--tag '["link-preview","snapshot","1",...]'` (repeatable) on `messages send`, with the relay or CLI applying the same validation the desktop writer applies. This is the most generally useful and the smallest change: every other NIP this project adds later becomes reachable from the CLI at the same time.

**Alternatives considered**

1. A purpose-built `--link-preview <json>` flag, if arbitrary tags are considered too broad a surface to expose. More conservative, but each future tag kind then needs its own flag.
2. Exposing `link_preview_tags::append`-equivalent validation from the SDK so a client can build the tag and hand it to `build_message`.
3. Workaround in use meanwhile: `messages send --file <thumbnail>`, which uploads an image and appends `` to the body. It shows a picture, but it edits the message body rather than annotating it, and it produces no title, site or description.

**Additional context**

Relay `0.2.1`; CLI built from `main` as of 2026-08-12; checked again against `main` on 2026-08-23 (`SendMessageParams` unchanged).

Duplicates: none found (searched open issues and PRs for link-preview / tag / messages send).

Start in crates/buzz-cli/src/commands/messages.rs at the messages send parameter and tag-building flow, then compare the validation in desktop/src-tauri/src/link_preview_tags.rs with parsing in desktop/src/shared/lib/linkPreviewSnapshot.ts. Trace the existing upload file path and message-send entry point; done means repeatable --tag JSON reaches the event while applying the same link-preview validation.

Written by the indexing model from the issue text.