Summary

extensions/slack plugin outbound sendMedia drops mediaLocalRoots before calling sendMessageSlack.

Repro (deterministic)

1. Open extensions/slack/src/channel.ts.
2. In outbound sendMedia, note options passed to send(...) include mediaUrl, threadTs, accountId, but not mediaLocalRoots.
3. Open src/slack/send.ts; sendMessageSlack supports mediaLocalRoots and forwards it to media loading/upload paths.

Expected

mediaLocalRoots provided to outbound sendMedia should be forwarded to sendMessageSlack so local media paths allowed by workspace roots resolve.

Actual

mediaLocalRoots is dropped in the extension adapter.

Root cause

extensions/slack/src/channel.ts outbound.sendMedia omits mediaLocalRoots from the outbound options payload.