Force changelog on pull requests

.github/workflows/changelog.yml

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+name: Changelog
+
+on:
+  pull_request:
+    branches: [master]
+    types: [opened, synchronize, reopened, labeled, unlabeled]
+
+jobs:
+  has_changelog:
+    runs-on: ubuntu-latest
+    # Opt-out: PRs labeled 'No changelog' skip the check entirely
+    # (docs-only, dependency bumps, CI tweaks).
+    if: ${{ !contains(github.event.pull_request.labels.*.name, 'No changelog') }}
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Check that the PR adds a changelog entry
+        run: |
+          git fetch origin "${{ github.base_ref }}"
+          head_sha="${{ github.event.pull_request.head.sha || 'HEAD' }}"
+          changed="$(git diff --name-only "origin/${{ github.base_ref }}"..."$head_sha" -- changelog/)"
+          if [ -z "$changed" ]; then
+            echo "::error::No changelog entry found. Add a file under changelog/ (see changelog/README.md), or add the 'No changelog' label if this PR is docs/CI/deps-only."
+            exit 1
+          fi
+          echo "Changelog entry present:"
+          echo "$changed"

changelog/README.md

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+# Changelog entries
+
+Every PR targeting `master` must add at least one file under this directory,
+describing the user-facing change. A GitHub Actions workflow
+(`.github/workflows/changelog.yml`) enforces this on every pull request.
+
+## Convention
+
+- One file per change. Name it after the Phabricator ticket (`T50530`) or, if
+  there is no ticket, a short descriptive slug (`access-log`).
+- File content is one or more bullet lines describing the change, e.g.:
+  ```
+  - [T50530] Bump sentry version
+  ```
+- Entries are collated into the top-level `CHANGELOG.md` at release time.
+
+## Skipping the check
+
+For docs-only, dependency-bump, or CI-only PRs that need no changelog entry,
+add the `no-changelog` label to the PR. The workflow will skip the check.
+
+See `.github/workflows/changelog.yml` for the exact enforcement logic.