ci: support stacked PRs in docs update cron job

[devin-ai-integration]

ci: support stacked PRs in docs update cron job

Summary

Updates the devin_update_docs cron workflow so that when an open docs update PR already exists from a previous run, the next Devin session is instructed to stack its PR on top of the existing one (branching from it and targeting it as base) instead of creating a separate PR against main.

A new "Find existing docs update PR" step uses gh pr list to search for open PRs authored by app/devin-ai-integration whose title matches docs.*update|update.*docs and branch starts with devin/. If found, the branch name and PR number are passed into the Devin prompt via Jinja2 template variables.

Also closed two outdated docs update PRs (#3674, #3615) that were stale from previous cron runs.

Review & Testing Checklist for Human

• Verify the jq filter logic: sort_by(.number) | last // empty — confirm this correctly picks the most recent PR and that // empty produces an empty string (not "null") when no match
• Verify the bash condition if [ -n "$DOCS_PR" ] && [ "$DOCS_PR" != "null" ] handles both the empty-string and JSON-null cases correctly
• Check the title regex docs.*update|update.*docs — consider whether it's too broad (could match unrelated PRs) or too narrow (could miss legitimate docs cron PRs). May want to filter by tag docs_update instead if the Devin API tags PRs
• Confirm that Devin reliably follows the stacked PR instructions (checkout existing branch → create new branch from it → set base branch). This is prompt-based and not mechanically enforced
• Consider triggering a manual workflow_dispatch run to test end-to-end with and without an existing open PR

Notes

• Requested by: @ComputelessComputer
• Devin session

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[devin-ai-integration]

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