n0-slack-history-mirror

Slack is the slowest part of the n0 sync, most of its three minutes. Every run
lists each channel's history, and whenever anything in a month changes, the
month is rebuilt by refetching the replies of every thread in it. So one new
message in a busy month costs dozens of serial `conversations.replies` calls
at Tier 3 pacing.

Let's do the raw reconciler layer, at 999 page size:

- Keep the Slack messages and thread replies in the project's store exactly as
  Slack returns them: every message and reply, bot posts included, with all
  of their fields. Nothing gets decided at fetch time, so when we later change
  what a month document shows (we do want bot posts in there eventually) it is
  a re-render, not a refetch of the workspace.
- Build the month documents from what is stored, so that a thread's replies
  are fetched again only when its reply count or latest reply has changed.
  Listing each channel's history on every run stays: that listing is where
  those signals come from.
- Month documents must come out byte-for-byte as they do today.
- Months that Slack's retention no longer shows must keep working the way they
  do now.
- Page Slack's history and replies listings at 999.

## Environment

PostgreSQL on localhost:5432 and the local database endpoint the app's
driver uses are running when you start, and the dev env files point at the
dev database.