feat: automate CHANGELOG.md updates from GitHub Release notes (#224)

`CHANGELOG.md` had no automated update mechanism — entries had to be
added manually or not at all. This wires the existing `gh release create
--generate-notes` output directly into `CHANGELOG.md` on every stable
release.

## What changed

### `.github/release.yml` (new)
Configures GitHub's auto-generated release note categories via PR
labels: `breaking`, `feat`/`enhancement`, `fix`/`bug`,
`docs`/`documentation`. Unlabelled PRs fall into "Other Changes";
`ignore-for-release` suppresses an entry entirely.

### `CHANGELOG.md`
Replaced the placeholder with a Keep a Changelog header and an HTML
comment marker that the workflow targets for insertion:
```
<!-- New entries are prepended automatically by the release workflow. -->
```

### `.github/workflows/release.yml` — `update-changelog` job
New job, runs after `release`, stable branches only (`is-release ==
'true'`):
1. Fetches the release body: `gh release view <version> --json body --jq
'.body'`
2. Inserts `## <version> — YYYY-MM-DD` + release notes immediately after
the comment marker using `awk`, leaving the rest of the file intact
3. Commits and pushes back to `release/X.Y` (same pattern as the
existing `promote-api-files` job)

`merge-to-main` gains `update-changelog` in its `needs`, so the PR to
`main` always includes the updated file.

### `.github/PULL_REQUEST_TEMPLATE.md`
Added checklist item: apply one of the release-note label categories
(`breaking` / `feat` / `fix` / `docs`).

### `docs/workflows.md`
Added `update-changelog` row to the `release.yml` job table.

Author: Copilot

.github/PULL_REQUEST_TEMPLATE.md

@@ -16,3 +16,4 @@
 - [ ] XML doc comments updated for all public API changes
 - [ ] `dotnet format` run with no issues
 - [ ] README and `/samples` updated — if public API changed
+- [ ] PR label added (`breaking` / `feat` / `fix` / `docs`) — for changelog categorization

.github/release.yml

@@ -0,0 +1,23 @@
+changelog:
+  exclude:
+    labels:
+      - ignore-for-release
+  categories:
+    - title: Breaking Changes
+      labels:
+        - breaking
+    - title: New Features
+      labels:
+        - feat
+        - enhancement
+    - title: Bug Fixes
+      labels:
+        - fix
+        - bug
+    - title: Documentation
+      labels:
+        - docs
+        - documentation
+    - title: Other Changes
+      labels:
+        - '*'

.github/workflows/release.yml

@@ -361,9 +361,68 @@ jobs:
           is-preview: ${{ env.is-preview }}
           notes-start-tag: ${{ steps.determine-notes-start-tag.outputs.notes-start-tag }}
 
+  update-changelog:
+    name: 'Update CHANGELOG.md'
+    needs: [workflow-variables, release, versioning]
+    if: ${{ needs.workflow-variables.outputs.is-release == 'true' }}
+    runs-on: ubuntu-latest
+    env:
+      GH_TOKEN: ${{ github.token }}
+      release-version: ${{ needs.versioning.outputs.release-version }}
+    steps:
+      - name: 'Checkout ${{ github.ref }}'
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ github.ref }}
+
+      - name: 'Fetch release notes'
+        shell: bash
+        run: |
+          gh release view ${{ env.release-version }} --json body --jq '.body' > /tmp/release-notes.txt
+
+      - name: 'Prepend entry to CHANGELOG.md'
+        shell: bash
+        run: |
+          release_date=$(date -u +%Y-%m-%d)
+          {
+            echo "## ${{ env.release-version }} — ${release_date}"
+            echo ""
+            cat /tmp/release-notes.txt
+            echo ""
+          } > /tmp/new-entry.txt
+          awk '
+            /<!-- New entries are prepended automatically by the release workflow. -->/ {
+              print
+              print ""
+              while ((getline line < "/tmp/new-entry.txt") > 0) print line
+              close("/tmp/new-entry.txt")
+              next
+            }
+            { print }
+          ' CHANGELOG.md > /tmp/changelog-new.md
+          mv /tmp/changelog-new.md CHANGELOG.md
+
+      - name: 'Configure git identity'
+        uses: './.github/actions/git/configure-identity'
+
+      - name: 'Commit and push CHANGELOG.md'
+        shell: bash
+        run: |
+          git add CHANGELOG.md
+          git diff --staged --quiet || (
+            git commit -m "Update CHANGELOG.md for ${{ env.release-version }}" &&
+            git pull --rebase origin ${{ github.ref_name }} &&
+            git push
+          )
+
+      - name: 'Write changelog summary'
+        shell: bash
+        run: |
+          echo "✅ CHANGELOG.md updated for **${{ env.release-version }}**." >> $GITHUB_STEP_SUMMARY
+
   merge-to-main:
     name: 'Merge ${{ github.ref_name }} into main'
-    needs: [workflow-variables, release, versioning]
+    needs: [workflow-variables, release, versioning, update-changelog]
     if: ${{ needs.workflow-variables.outputs.is-release == 'true' }}
     runs-on: ubuntu-latest
     permissions:

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Changelog
 
-Release history is tracked via GitHub Releases.
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
+and release notes are generated automatically from merged pull requests.
 
 See all releases at: <https://github.com/petesramek/polyline-algorithm-csharp/releases>
+
+<!-- New entries are prepended automatically by the release workflow. -->

docs/workflows.md

@@ -87,6 +87,7 @@ Version bumping runs independently via `bump-version.yml` (manual trigger).
 | `pack` | `versioning`, `build` | Packages binaries |
 | `publish-package` | `pack` | Publishes to NuGet.org (Production environment) |
 | `create-release` | `versioning`, `publish-package` | Creates a git tag + GitHub release with auto-generated notes |
+| `update-changelog` | `release` | Fetches release notes and prepends a new entry to `CHANGELOG.md` — stable releases only |
 | `generate-docs` | `versioning` | Builds DocFX site |
 | `publish-docs` | `generate-docs` | Deploys to GitHub Pages |