fix(desktop): resolve activity feed showing channel UUID instead of message content

[wpfleger96]

Stops extracting agent-sent message content from shell command text and fetches it from the relay event instead, eliminating the case where the Activity feed rendered a bare channel UUID (or literal shell syntax) as the message body.

Problem

For buzz messages send --content - (stdin) forms — including printf pipes — the classifier's arg-parsing fell back to extractBuzzCliObjectPreview, which surfaces the first available flag value (--channel, --event, etc). Since --channel is present on every send, the UUID rendered as if it were the message text. Forms like --content "$(cat file)" or --content "$MESSAGE" had a related symptom: the literal shell syntax rendered as the message body instead of the content that was actually sent.

Fix

parseBuzzCliCommand (agentSessionToolClassifier.ts) now only extracts a --content value for messages.send when it is a statically exact inline string — any value containing $ or a backtick anywhere (stdin, command substitution, variable expansion) yields null and never falls through to the object-preview fallback. extractSimpleEchoPipeContent and its helper are removed — no shell-form-specific parsing left to extend.

When arg-side content is null, the useSentMessageBody hook (AgentSessionToolItem/useSentMessageBody.ts) fetches the real event body from the relay via getEventById, keyed off the message ID already recovered from the tool result by getSentMessageLink. The event is nostr-immutable, so the query is cached with staleTime: Infinity. CompactMessageSummary renders inline content, then fetched content, then the existing "Message content unavailable." placeholder — never a raw CLI flag value or shell fragment.