refactor(upgrade): rewrite as composite action

Replace the Node.js action (TypeScript + tsup bundle) with a composite
action to align with the repo's architectural pattern where actions are
thin shell wrappers around CLI commands.

- Rewrite action.yml from `using: node20` to `using: composite`
- Add scripts/snapshot.js for version capture (plain JS, no npm deps)
- Add scripts/diff.js for diff computation + PR content generation
- Reuse resolve-cli composite action for CLI version resolution
- Use gh CLI for PR management instead of @actions/github
- Delete all TypeScript source, tests, dist bundle, and build config
- Remove upgrade from pnpm workspace (no package.json needed)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: 2 people

actions/upgrade/action.yml

@@ -18,16 +18,133 @@ inputs:
 outputs:
   run-id:
     description: "UUIDv4 for this upgrade run"
+    value: ${{ steps.run-id.outputs.run-id }}
   pr-url:
     description: "URL of the created or updated upgrade PR (empty if no changes)"
+    value: ${{ steps.manage-pr.outputs.pr-url }}
   cli-upgraded:
     description: "Whether the Fern CLI version was upgraded (true/false)"
+    value: ${{ steps.diff.outputs.cli-upgraded }}
   generators-upgraded:
     description: "JSON array of {generator, from, to} for each upgraded generator"
+    value: ${{ steps.diff.outputs.generators-upgraded }}
 
 runs:
-  using: "node20"
-  main: "dist/index.js"
+  using: composite
+  steps:
+    - name: Resolve Fern CLI
+      id: cli
+      uses: fern-api/actions/resolve-cli@main
+      with:
+        version: ${{ inputs.version }}
+
+    - name: Set run ID
+      id: run-id
+      shell: bash
+      run: |
+        if [ -z "${FERN_RUN_ID}" ]; then
+          FERN_RUN_ID=$(node -e "console.log(require('crypto').randomUUID())")
+          echo "FERN_RUN_ID=${FERN_RUN_ID}" >> "$GITHUB_ENV"
+          echo "Generated new FERN_RUN_ID: ${FERN_RUN_ID}"
+        else
+          echo "Inheriting existing FERN_RUN_ID: ${FERN_RUN_ID}"
+        fi
+        echo "run-id=${FERN_RUN_ID}" >> "$GITHUB_OUTPUT"
+
+    - name: Capture before versions
+      id: before
+      shell: bash
+      run: |
+        SNAPSHOT=$(node "${{ github.action_path }}/scripts/snapshot.js")
+        echo "snapshot=$SNAPSHOT" >> "$GITHUB_OUTPUT"
+        echo "Captured before snapshot: $SNAPSHOT"
+
+    - name: Run fern upgrade
+      shell: bash
+      env:
+        FERN_TOKEN: ${{ inputs.fern-token }}
+      run: |
+        echo "Running fern upgrade..."
+        ${{ steps.cli.outputs.fern-cmd }} upgrade --yes || \
+          echo "::warning::fern upgrade exited with non-zero code"
+
+    - name: Run fern generator upgrade
+      shell: bash
+      env:
+        FERN_TOKEN: ${{ inputs.fern-token }}
+      run: |
+        echo "Running fern generator upgrade..."
+        ${{ steps.cli.outputs.fern-cmd }} generator upgrade --yes || \
+          echo "::warning::fern generator upgrade exited with non-zero code"
+        # TODO: FER-9669 — Check automation config before running upgrades
+
+    - name: Compute upgrade diff
+      id: diff
+      shell: bash
+      env:
+        BEFORE_SNAPSHOT: ${{ steps.before.outputs.snapshot }}
+      run: node "${{ github.action_path }}/scripts/diff.js"
+
+    - name: Push changes and manage PR
+      id: manage-pr
+      if: steps.diff.outputs.has-changes == 'true'
+      shell: bash
+      env:
+        GH_TOKEN: ${{ inputs.github-token }}
+        GITHUB_TOKEN: ${{ inputs.github-token }}
+        PR_TITLE: ${{ steps.diff.outputs.pr-title }}
+        COMMIT_MSG: ${{ steps.diff.outputs.commit-msg }}
+      run: |
+        BRANCH="fern/upgrade"
+
+        # Configure git
+        git config user.name "github-actions[bot]"
+        git config user.email "github-actions[bot]@users.noreply.github.com"
+
+        # Get default branch
+        DEFAULT_BRANCH=$(gh api "repos/${{ github.repository }}" --jq '.default_branch')
+        echo "Default branch: $DEFAULT_BRANCH"
+
+        # Warn if overwriting existing branch
+        if git ls-remote --heads origin "$BRANCH" | grep -q "$BRANCH"; then
+          echo "::warning::Existing $BRANCH branch will be overwritten (clean-slate strategy)"
+        fi
+
+        # Clean-slate branch from default branch HEAD
+        git fetch origin "$DEFAULT_BRANCH"
+        git checkout -B "$BRANCH" "origin/$DEFAULT_BRANCH"
+
+        # Stage all changes
+        git add -A
+        if git diff --cached --quiet; then
+          echo "No changes to commit"
+          echo "pr-url=" >> "$GITHUB_OUTPUT"
+          exit 0
+        fi
+
+        # Commit and force push
+        git commit -m "$COMMIT_MSG"
+        git push --force origin "$BRANCH"
+
+        # Write PR body to temp file to avoid shell escaping issues
+        cat > /tmp/pr-body.md << 'PREOF'
+        ${{ steps.diff.outputs.pr-body }}
+        PREOF
+
+        # Create or update PR
+        EXISTING_PR=$(gh pr list --head "$BRANCH" --state open --json number --jq '.[0].number // empty')
+        if [ -n "$EXISTING_PR" ]; then
+          gh pr edit "$EXISTING_PR" --title "$PR_TITLE" --body-file /tmp/pr-body.md
+          PR_URL=$(gh pr view "$EXISTING_PR" --json url --jq '.url')
+          echo "Updated existing PR: $PR_URL"
+        else
+          PR_URL=$(gh pr create --head "$BRANCH" --base "$DEFAULT_BRANCH" \
+            --title "$PR_TITLE" --body-file /tmp/pr-body.md)
+          echo "Created new PR: $PR_URL"
+        fi
+
+        echo "pr-url=$PR_URL" >> "$GITHUB_OUTPUT"
+        # TODO: FER-9668 — Emit upgrade telemetry
 
 branding:
   icon: "refresh-cw"