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.

discourse-release/action.yml

@@ -0,0 +1,37 @@
+name: discourse-release
+description: >-
+  Announce a published Gitea release on the Coffey Labs community forum, in the
+  repo's Announcements category (release-map.json). Safe to re-run: a release
+  is announced at most once.
+inputs:
+  api-key:
+    description: release-bot's forum API key (scope topics:write only)
+    required: true
+  discord-webhook:
+    description: optional Discord webhook URL; when set, the announcement is also posted there
+    required: false
+    default: ""
+  forum:
+    description: forum base URL
+    required: false
+    default: https://community.coffeylabs.org
+runs:
+  using: composite
+  steps:
+    - shell: sh
+      env:
+        API_KEY: ${{ inputs.api-key }}
+        DISCORD_WEBHOOK: ${{ inputs.discord-webhook }}
+        FORUM: ${{ inputs.forum }}
+        REPO: ${{ github.repository }}
+        EVENT_PATH: ${{ github.event_path }}
+        ACTION_PATH: ${{ github.action_path }}
+        MAP: ${{ github.action_path }}/release-map.json
+      run: |
+        set -eu
+        # The job image may be bare; the script needs bash, curl and jq.
+        if ! command -v curl >/dev/null || ! command -v jq >/dev/null || ! command -v bash >/dev/null; then
+          if command -v apk >/dev/null; then apk add --no-cache -q bash curl jq ca-certificates >/dev/null
+          else apt-get update -qq >/dev/null && apt-get install -y -qq --no-install-recommends bash curl jq ca-certificates >/dev/null; fi
+        fi
+        exec bash "$ACTION_PATH/announce.sh"