discourse-release: announce published releases on the community forum

A composite action: reads the release from the event file with jq, looks up
the repo's Announcements category (release-map.json) and posts as
release-bot. One topic per repo+tag via the topic external_id, so a re-run
is a no-op. Optional Discord webhook.

README.md

@@ -18,3 +18,32 @@ Pin every use by full commit SHA.
 Works in any job image with a POSIX shell. Installs git with apk or apt-get
 if the image has none, and clones from the runner's internal Gitea address
 (`CI_SERVER_INTERNAL`) so CI traffic never crosses Cloudflare.
+
+## discourse-release
+
+Announces a published release on the community forum, in the repo's
+Announcements category. Add to a project repo as `.gitea/workflows/announce.yml`:
+
+```yaml
+on:
+  release:
+    types: [published]
+jobs:
+  announce:
+    runs-on: light
+    steps:
+      - uses: coffey-labs/actions/discourse-release@<sha>
+        with:
+          api-key: ${{ secrets.DISCOURSE_RELEASE_KEY }}
+          discord-webhook: ${{ secrets.DISCORD_RELEASE_WEBHOOK }}   # optional
+```
+
+- The repo must be listed in `discourse-release/release-map.json`, the one
+  place that maps repos to forum categories.
+- `DISCOURSE_RELEASE_KEY` is an org-level secret (coffey-labs and inbuxa)
+  holding release-bot's key, which can only create posts.
+- Re-running is safe: each repo+tag gets one topic, enforced by the forum.
+- Release notes are passed to the forum through `jq` only, so quotes,
+  backticks and multi-line Markdown arrive exactly as written.
+- To turn it off for a repo, delete its `announce.yml`; for everyone, remove
+  the org secret.