[Docs] Sync Slack audio and Gemini guidance

[roomote-community]

Created by Roomote. Follow up by mentioning @roomote, in the web UI, or in Discord.

What changed

• document Slack audio attachment transcription, including supported formats, the 20 MiB limit, first-attachment behavior, model requirements, and warning cases
• update the Google Gemini guide to identify Gemini 3.6 Flash as the default recommendation across roles

Why this change was made

These user-facing behaviors merged without complete matching public documentation. The update keeps the provider guides aligned with the current shipped behavior.

Impact

Users and operators can now find accurate guidance for Slack audio messages and current Google Gemini model recommendations. There is no product behavior change.

Screenshots

[roomote-community]

1 issue outstanding. See task

• apps/docs/providers/communications/slack.mdx:159: The multiple-audio warning is absent when the first audio attachment is unsupported or oversized.

_{Reviewed bcef4bf}

[roomote-community]

This condition is not unconditional. In processSlackAttachments, an unsupported or oversized first audio attachment returns at lines 145-161 before the audioFiles.length > 1 block at line 201, so multiple attachments in either case produce only the unsupported/size warning. Qualify this sentence or change the handler so the docs remain accurate.