Adds a new 'Attachment vision reference' section to docs/channels/slack.md covering:
- Supported media types (images, PDFs, files) and their vision pipeline status
- Inbound pipeline flow (download → media store → vision → context injection)
- Thread-root attachment inheritance via thread_ts fallback
- Multi-attachment parallel processing
- Size and format limits
- Known limits table (expired URLs, missing vision model, large images, forwarded attachments)
- Links to epic, regression tests, and live verification issues
Addresses #51355Closes#51355