#11898: Fix for issue #11895

Fixing Nostr Channel Message Sending Issue <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR updates the Nostr channel plugin to add an explicit `outbound.sendMedia` handler that rejects media sends for Nostr DMs (which are text-only under NIP-04). The change lives in `extensions/nostr/src/channel.ts` alongside the existing `sendText` implementation in the plugin’s `outbound` section. <h3>Confidence Score: 4/5</h3> - Mostly safe to merge, but the new media-send behavior may cause avoidable runtime errors if invoked. - Change is small and localized, but adding a `sendMedia` that always throws can crash callers if they invoke it without checking `capabilities.media` (or if interface expectations changed). - extensions/nostr/src/channel.ts <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} **Context used:** - Context from `dashboard` - CLAUDE.md ([source](https://app.greptile.com/review/custom-context?memory=fd949e91-5c3a-4ab5-90a1-cbe184fd6ce8)) - Context from `dashboard` - AGENTS.md ([source](https://app.greptile.com/review/custom-context?memory=0d0c8278-ef8e-4d6c-ab21-f5527e322f13)) <!-- /greptile_comment -->