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/release-map.json

@@ -0,0 +1,14 @@
+{
+  "_comment": "Gitea repo -> forum Announcements category (parent/child slug). The one place this mapping lives; adding a project is one line here.",
+  "inbuxa/inbuxa-server": "inbuxa/announcements",
+  "inbuxa/ihasmail-inbuxa": "inbuxa/announcements",
+  "inbuxa/inbuxa-admin": "inbuxa/announcements",
+  "inbuxa/inbuxa-installer": "inbuxa/announcements",
+  "coffey-labs/ihasmail": "ihasmail/announcements",
+  "coffey-labs/ihasmail-oneshot": "ihasmail/announcements",
+  "coffey-labs/ihasvpn": "ihasvpn/announcements",
+  "coffey-labs/cairnobs": "cairn-obs/announcements",
+  "coffey-labs/stalwart-migrator": "coffey-labs/announcements",
+  "coffey-labs/SysAdminAutomation": "coffey-labs/announcements",
+  "coffey-labs/ubuntu2mint": "coffey-labs/announcements"
+}