ci(autoupdate): drop synchronize trigger, keep draft PRs updated

Narrow the pull_request trigger to opened/reopened/ready_for_review so
per-commit pushes to open PRs no longer storm the runners via a
rebase-of-all-PRs (PR_FILTER: all) on every synchronize event. Pin
PR_READY_STATE to "all" so draft PRs remain in scope for updates
triggered by push (main advancing) and opened.

Closes #262

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.github/workflows/autoupdate.yml

@@ -18,6 +18,7 @@ on:
   push:
     branches: [main]
   pull_request:
+    types: [opened, reopened, ready_for_review]
     branches: [main]
 
 permissions:
@@ -39,4 +40,5 @@ jobs:
         env:
           GITHUB_TOKEN: ${{ secrets.AUTOUPDATE_TOKEN || secrets.GITHUB_TOKEN }}
           PR_FILTER: "all"
+          PR_READY_STATE: "all"
           MERGE_CONFLICT_ACTION: "ignore"