README: the Slack module joins the list

Every shipped module had an entry under "Included modules" except Slack,
which wasn't mentioned anywhere in the README — so the one module that can
send as the campus was also the one nobody could read about.

Its entry now sits with the others, in their shape: what it does, the tiles
it offers in the ＋ picker (one transcript per configured channel plus the
two single send surfaces), its four admin groups and its tile settings —
all taken from modules/slack/module.json and client.js.

The testing paragraph listed the ProdCom, ProPresenter and PCO mocks but
not the Slack one, though CLAUDE.md has documented it all along; it is now
there with the SLACK_API_BASE line it needs.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -265,6 +265,29 @@ below. Nothing is installed without someone clicking.
   (gear menu): the top-bar text (placeholders such as
   `{series} • {part} | {serviceType} • {date}`), colors, text size, and a
   show/hide checklist for every detail.
+- **Slack** — live transcripts of this location's Slack channels, plus
+  PIN-protected sending as the location. The ＋ picker lists one transcript
+  tile per configured channel, titled `#channel`, and two send surfaces (one
+  of each per dashboard): **Send message**, a free-text box with the admin's
+  quick emojis one press away, and **Quick replies**, a grid of preset
+  buttons. Both post to the channel of whichever transcript was clicked last
+  — its bottom edge lights up in the accent color — and to the first
+  configured channel until one is clicked. Messages arrive with avatars,
+  names and times, Slack's inline formatting and the workspace's custom
+  emoji; a right-click on one opens the reaction menu, and text size and
+  jump-to-latest sit in the title bar. Nothing leaves the machine until the
+  PIN is entered: one PIN unlocks every Slack tile on that dashboard page
+  until the auto-lock idles out, a reload asks again, and the lock button on
+  a send tile's title bar ends it early.
+  Admin config is in four groups: **Slack account** (the location account's
+  User OAuth Token — `xoxp-…`, so messages are literally from the campus —
+  or a bot token plus "Send as"), **Channels** (the joined channels to
+  follow, one transcript tile each), **Transcript** (refresh interval,
+  messages kept, thread replies) and **Sending** (the switch on the group's
+  title row, a 4–8 digit PIN, auto-lock minutes, quick replies one per line
+  as `Label | Message` — add `| #channel` to bind one to a channel — and up
+  to eight quick emojis).
+  Tile settings: avatars, timestamps, names and channel notices.
 - **Clock** — a big booth clock; also the smallest possible module and the
   reference for module authors.
 
@@ -272,10 +295,12 @@ To develop modules against mocks instead of real gear:
 `node tools/prodcom-mock.js` (ProdCom on `127.0.0.1:24480`),
 `node band-lineup-display/tmp/pp-mock.js 1599` (ProPresenter slides on
 `127.0.0.1:1599`), `node tools/pp-timers-mock.js 1600` (ProPresenter
-timers + timecode on `127.0.0.1:1600`) and `node tools/pco-mock.js`
+timers + timecode on `127.0.0.1:1600`), `node tools/pco-mock.js`
 (Planning Center Services on `127.0.0.1:24700` — run ProdDash with
-`PCO_PLAN_API_BASE=http://127.0.0.1:24700`). See the testing section of the
-[module guide](docs/MODULE-GUIDE.md).
+`PCO_PLAN_API_BASE=http://127.0.0.1:24700`) and
+`node tools/slack-mock.js 24716` (the Slack Web API on `127.0.0.1:24716` —
+run ProdDash with `SLACK_API_BASE=http://127.0.0.1:24716/api`). See the
+testing section of the [module guide](docs/MODULE-GUIDE.md).
 
 ## Install as a desktop web app