docs+ci: team communication guide and optional Slack notifications

.github/COMMUNICATION.md

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+# Team Communication & Workflow
+
+This guide describes how the Tuple team communicates and how work flows from a
+conversation to a shipped change. It covers the Slack channel, the GitHub
+integration, and the Claude assistant.
+
+## Where we talk
+
+| Channel            | Purpose                                                         |
+| ------------------ | -------------------------------------------------------------- |
+| Slack `#tuple-vst` | Day-to-day coordination, GitHub notifications, Claude requests |
+| Discord            | Community support and user questions (see the README)          |
+| GitHub issues/PRs  | Anything that needs a durable record or code review            |
+
+## GitHub ↔ Slack
+
+The `#tuple-vst` channel is subscribed to GitHub notifications:
+
+```
+/github subscribe c0remusic/tuple issues pulls commits releases deployments
+```
+
+That posts issue, pull-request, commit, release, and deployment activity
+straight into the channel. To change what is delivered:
+
+```
+/github subscribe   c0remusic/tuple <feature> [<feature> ...]
+/github unsubscribe c0remusic/tuple <feature> [<feature> ...]
+/github subscribe list features
+```
+
+Available features include `issues`, `pulls`, `commits`, `releases`,
+`deployments`, `reviews`, and `comments`.
+
+### Optional: custom Slack notifications
+
+`.github/workflows/slack-notify.yml` posts two extra messages the GitHub app
+does not send by default:
+
+- a :rocket: message when a GitHub Release is published, and
+- a pass/fail message when the *Test Mac Installer* workflow finishes.
+
+It only runs if a `SLACK_WEBHOOK_URL` repository secret is set. To enable it:
+
+1. Create an Incoming Webhook in Slack and copy its URL.
+2. In GitHub, go to **Settings → Secrets and variables → Actions**
+   and add a secret named `SLACK_WEBHOOK_URL`.
+
+If the secret is absent the workflow is a no-op, so it is safe to merge before
+the webhook exists.
+
+## Working with Claude
+
+Claude (the Anthropic assistant) is in the channel and can pick up tasks.
+
+- **Ask for work in a thread.** Describe the change; mentioning Claude in a
+  thread copies that thread to the pull request as context.
+- **Branches.** Claude develops on `claude/<topic>` branches, one per task.
+- **Pull requests.** Claude opens a **draft** PR per branch, follows
+  `.github/PULL_REQUEST_TEMPLATE.md`, and links back to the originating Slack
+  thread in the description.
+- **Review.** A human reviews and merges. Claude watches its own PRs and
+  responds to CI failures and review comments in the thread.
+
+## Branch & PR conventions
+
+- One branch per task; keep it focused.
+- Commit messages follow the existing mixed-scope style (`feat(ui):`,
+  `fix(midi):`, `build:`, …). English or French are both fine, matching the
+  repo history.
+- Every PR follows `.github/PULL_REQUEST_TEMPLATE.md`
+  (Summary / Type of change / Checklist).
+- Link the Slack thread that prompted the change.
+
+## Release flow
+
+1. Bump `VERSION` and land the release commit on `main`.
+2. Publish a GitHub Release tagged `vX.Y.Z`.
+3. The GitHub Slack app announces it in `#tuple-vst`; if the webhook is
+   configured, the custom workflow adds a :rocket: message too.

.github/workflows/slack-notify.yml

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+name: Slack notifications
+
+on:
+  release:
+    types: [published]
+  workflow_run:
+    workflows: ["Test Mac Installer"]
+    types: [completed]
+
+# Posts extra Slack messages that the GitHub Slack app does not send by
+# default. No-op unless a SLACK_WEBHOOK_URL repository secret is configured.
+jobs:
+  notify:
+    runs-on: ubuntu-latest
+    env:
+      SLACK_WEBHOOK_URL: ${{ secrets.SLACK_WEBHOOK_URL }}
+    steps:
+      - name: Announce published release
+        if: ${{ github.event_name == 'release' && env.SLACK_WEBHOOK_URL != '' }}
+        env:
+          TAG: ${{ github.event.release.tag_name }}
+          URL: ${{ github.event.release.html_url }}
+        run: |
+          msg=":rocket: *Tuple ${TAG}* published — ${URL}"
+          jq -n --arg t "$msg" '{text:$t}' \
+            | curl -sS -X POST -H 'Content-type: application/json' --data @- "$SLACK_WEBHOOK_URL"
+
+      - name: Report installer test result
+        if: ${{ github.event_name == 'workflow_run' && env.SLACK_WEBHOOK_URL != '' }}
+        env:
+          CONCLUSION: ${{ github.event.workflow_run.conclusion }}
+          BRANCH: ${{ github.event.workflow_run.head_branch }}
+          URL: ${{ github.event.workflow_run.html_url }}
+        run: |
+          if [ "$CONCLUSION" = "success" ]; then icon=":white_check_mark:"; else icon=":x:"; fi
+          msg="${icon} *Test Mac Installer* ${CONCLUSION} on \`${BRANCH}\` — ${URL}"
+          jq -n --arg t "$msg" '{text:$t}' \
+            | curl -sS -X POST -H 'Content-type: application/json' --data @- "$SLACK_WEBHOOK_URL"