Add Slack channel extraction as a document source

[cameronhotchkies]

Problem

frfr currently only extracts facts from PDFs and Markdown files. A lot of institutional knowledge, decisions, and technical context lives in Slack conversations and threads — but there's no way to point frfr at a channel and extract structured facts from it.

Solution

Add Slack as a document source alongside PDFs and Markdown. A user points frfr at a Slack channel, and the system extracts facts from conversations and threads that can be queried with citations back to the original messages.

Acceptance Criteria

• A user can import a Slack channel into a session by providing a channel ID or URL
• Messages and thread replies are included in the extraction
• Slack-specific formatting (mentions, links, emoji) is preserved as readable text, not raw markup
• Extracted facts can be queried and answers cite back to specific Slack messages with permalinks
• Channels with 1,000 or fewer messages are ingested in full
• Channels with more than 1,000 messages default to the last 90 days
• The message cap (1,000) and lookback period (90 days) are configurable
• A user can specify a custom date range to override the automatic bounding