docs+ci: team communication guide and optional Slack notifications

[c0remusic]

Summary

Sets up the team communication workflow so Slack, GitHub, and Claude interoperate cleanly for the c0remusic/tuple repo.

• .github/COMMUNICATION.md — a guide covering where the team talks (Slack #tuple-vst, Discord, GitHub), the GitHub↔Slack /github subscribe integration, how to work with Claude, branch & PR conventions, and the release flow.
• .github/workflows/slack-notify.yml — an optional GitHub Actions workflow that posts two Slack messages the GitHub Slack app doesn't send by default: a 🚀 note when a Release is published, and a pass/fail note when the Test Mac Installer workflow finishes. It is a no-op unless a SLACK_WEBHOOK_URL repository secret is configured, so it is safe to merge before the webhook exists.

The doc was placed in .github/ (not docs/, which is gitignored in this repo) to sit alongside the existing process docs (CONTRIBUTING, SECURITY, CODE_OF_CONDUCT).

This branch was rebuilt on top of the current main (v1.5.0) so it contains only these two new files.

Type of change

• Documentation
• CI / tooling
• Bug fix
• New feature
• Breaking change

Checklist

• New files follow the repo's existing conventions
• Workflow is guarded so it fails safe (no-op) without the SLACK_WEBHOOK_URL secret
• slack-notify.yml validated as YAML
• SLACK_WEBHOOK_URL secret added in GitHub settings (required only to activate the notifications)

Setup to activate Slack notifications (optional)

1. Create an Incoming Webhook in Slack, copy its URL.
2. In GitHub → Settings → Secrets and variables → Actions, add a secret named SLACK_WEBHOOK_URL.

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Originating Slack thread: https://tuplegroupe.slack.com/archives/C0BLCSNRAVB/p1785284610582309?thread_ts=1785284610.582309&cid=C0BLCSNRAVB

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