feat: index meeting decisions and Slack exports from gcal-organizer and get-out

[jflowers]

Problem

Two sibling projects produce valuable project context that Dewey cannot currently search:

1. gcal-organizer — extracts structured decisions from meeting transcripts (Decisions Made, Decisions Deferred, Open Items)
2. get-out — exports Slack conversations (DMs, channels, group messages) to Google Docs

Both projects write output to Google Docs but not to the local filesystem. Dewey has no way to index this content.

Proposal

Once gcal-organizer (jflowers/gcal-organizer#12) and get-out (jflowers/get-out#4) add local markdown export, configure Dewey disk sources to index the exported content:

# .uf/dewey/sources.yaml
- id: disk-meeting-decisions
  type: disk
  name: meeting-decisions
  config:
    path: "~/.gcal-organizer/decisions"

- id: disk-slack-export
  type: disk
  name: slack-conversations
  config:
    path: "~/.get-out/export"

This enables agents to search meeting decisions and Slack conversations via semantic_search alongside specs, code, and documentation — completing the knowledge graph with operational context that currently only exists in Google Workspace.

Value

• An agent working on auth can find "sprint planning decided to switch to OAuth2" from meeting decisions
• An agent investigating a bug can find "alice mentioned this edge case in #design-decisions" from Slack
• dewey compile can synthesize meeting decisions with stored learnings to produce current-state knowledge articles

Dependencies

• feat: export extracted decisions as local markdown for Dewey indexing jflowers/gcal-organizer#12 — local markdown export of extracted decisions
• feat: write local markdown copies of exported conversations for Dewey indexing jflowers/get-out#4 — local markdown export of Slack conversations

Design

• Zero new code in Dewey — existing type: disk source handles this. Just add entries to sources.yaml.
• Composability preserved — if neither tool is installed or the export directories don't exist, Dewey logs "source directory not found" and skips gracefully.
• Trust tier: Indexed as authored (human-written content from meetings and Slack), not draft.