changelog: release notes from the commit messages, in every format a release needs

The commits since the previous tag of the same kind become notes.md, notes.txt, a
CHANGELOG.md of the last releases, an nfpm changelog for the deb and rpm, notes.json
and 500-character store notes; with release: 'true' they also go onto the tag's
release on the Releases tab, inside a marked block that leaves the rest of the text
alone. The same file reads the history from Gitea's API, for the packages publisher.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

changelog/action.yml

@@ -0,0 +1,61 @@
+name: Release notes
+description: What changed since the previous tag, from the commit messages, in every format a release needs; optionally on the Releases tab
+inputs:
+  prefix:
+    description: The tag prefix of this kind of release (v, app-v, server-v)
+    default: v
+  tag:
+    description: The tag being released; by default the tag that started the run, or everything since the newest tag
+    default: ''
+  name:
+    description: The product's name in headings; the repository's name by default
+    default: ''
+  strip:
+    description: A regular expression removed from the start of each subject
+    default: ''
+  packager:
+    description: Name <address> written into the deb and rpm changelogs
+    default: Arasaka di Hvozdikov Yevheniy <packages@arasaka.software>
+  history:
+    description: How many releases CHANGELOG.md and the package changelog carry
+    default: '20'
+  output:
+    description: The folder the notes are written to
+    default: release-notes
+  release:
+    description: "'true' also creates or updates this tag's release on the Releases tab with the notes"
+    default: 'false'
+  token:
+    description: A token that may write this repository's releases (only with release)
+    default: ${{ github.token }}
+outputs:
+  dir:
+    description: The folder with notes.md, notes.txt, CHANGELOG.md, changelog.yml, notes.json and store/en-US.txt
+    value: ${{ steps.notes.outputs.dir }}
+  notes:
+    description: The path of notes.md
+    value: ${{ steps.notes.outputs.notes }}
+  previous:
+    description: The previous tag, empty for a first release
+    value: ${{ steps.notes.outputs.previous }}
+runs:
+  using: composite
+  steps:
+    - id: notes
+      shell: bash
+      env:
+        RELEASE_TOKEN: ${{ inputs.token }}
+        NOTES_PREFIX: ${{ inputs.prefix }}
+        NOTES_TAG: ${{ inputs.tag }}
+        NOTES_NAME: ${{ inputs.name }}
+        NOTES_STRIP: ${{ inputs.strip }}
+        NOTES_PACKAGER: ${{ inputs.packager }}
+        NOTES_HISTORY: ${{ inputs.history }}
+        NOTES_OUTPUT: ${{ inputs.output }}
+        NOTES_RELEASE: ${{ inputs.release }}
+      run: |
+        args=(--repo . --prefix "$NOTES_PREFIX" --name "$NOTES_NAME" --strip "$NOTES_STRIP" --packager "$NOTES_PACKAGER" \
+          --history "$NOTES_HISTORY" --out "$NOTES_OUTPUT")
+        [ -n "$NOTES_TAG" ] && args+=(--tag "$NOTES_TAG")
+        [ "$NOTES_RELEASE" = "true" ] && args+=(--release)
+        python3 "${{ github.action_path }}/changelog.py" "${args[@]}"