Add Slack internal processing workflow - #27

Summary

• add a Slack call-digest starter that turns an uploaded transcript into a structured summarize → extract digest
• keep intake, status, and digest cards in the originating mention thread
• reuse the shared Corbits Tag workspace inherited from the stacked Slack starters

Stack

This PR is stacked directly on #25 and targets its head branch, scout-fact-check.

The diff against that branch only adds starter/slack-internal-processing; it does not add another Corbits Tag checkout or repeat shared vendor infrastructure.

Commits

1. Scaffold the Slack internal processing package
2. Add call digest types and strict parser
3. Define summarize and extract digest workflow
4. Render Slack intake and digest cards
5. Drive transcript intake through Slack sessions
6. Mount the Corbits Tag Slack entrypoint
7. Cover digest parsing and session regressions
8. Document Slack internal processing setup

Verification

• bun test — 14 passed, 0 failed
• bun run typecheck
• bun run start --help
• bun build src/cli.ts --target=bun --outdir /tmp/slack-internal-processing-build

Live Slack verification was not repeated for this change.