ci: TRAC-881 Hook up Linear releases via linear-release-action

Sync every push to canary/integrations-makeswift into Linear's "Catalyst
Release" pipeline via linear/linear-release-action, so merged issues get
attached to the currently open release as PRs land. Once changesets
actually publishes a new @bigcommerce/catalyst-core or
@bigcommerce/catalyst-makeswift version, complete that release using the
GitHub Release notes changesets already generated for the tag.

Refs TRAC-881

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

Author: chanceaclark

.github/workflows/changesets-release.yml

@@ -21,6 +21,9 @@ jobs:
     steps:
       - name: Checkout Repo
         uses: actions/checkout@v4
+        with:
+          # Full history required for linear-release-action to scan commits.
+          fetch-depth: 0
 
       - name: Setup pnpm
         uses: pnpm/action-setup@v3
@@ -39,6 +42,12 @@ jobs:
         env:
           CLI_SEGMENT_WRITE_KEY: ${{ secrets.CLI_SEGMENT_WRITE_KEY }}
 
+      - name: Sync Linear release
+        uses: linear/linear-release-action@v0
+        with:
+          access_key: ${{ secrets.LINEAR_ACCESS_KEY }}
+          command: sync
+
       - name: Create Release Pull Request or Publish to npm
         id: changesets
         uses: changesets/action@v1
@@ -51,3 +60,34 @@ jobs:
           commit: "Version Packages (`${{ github.ref_name }}`)"
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Resolve headline package
+        id: headline
+        if: steps.changesets.outputs.published == 'true'
+        run: |
+          PACKAGE_NAME="@bigcommerce/catalyst-core"
+          if [ "${{ github.ref_name }}" = "integrations/makeswift" ]; then
+            PACKAGE_NAME="@bigcommerce/catalyst-makeswift"
+          fi
+          VERSION=$(echo '${{ steps.changesets.outputs.publishedPackages }}' | jq -r --arg name "$PACKAGE_NAME" '.[] | select(.name == $name) | .version')
+          if [ -z "$VERSION" ]; then
+            echo "found=false" >> "$GITHUB_OUTPUT"
+          else
+            echo "found=true" >> "$GITHUB_OUTPUT"
+            echo "tag=$PACKAGE_NAME@$VERSION" >> "$GITHUB_OUTPUT"
+          fi
+
+      - name: Fetch release notes
+        if: steps.headline.outputs.found == 'true'
+        run: gh release view "${{ steps.headline.outputs.tag }}" --json body -q .body > /tmp/release-notes.md
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Complete Linear release
+        if: steps.headline.outputs.found == 'true'
+        uses: linear/linear-release-action@v0
+        with:
+          access_key: ${{ secrets.LINEAR_ACCESS_KEY }}
+          command: complete
+          version: ${{ steps.headline.outputs.tag }}
+          release_notes: /tmp/release-notes.md