Add monthly SRE AI-readiness review workflow

[cainelli]

What

Adds a caller workflow (.github/workflows/sre-ai-readiness.yml) that invokes the shared reusable workflow from getyourguide/sre-actions (merged in PR #28, pinned to commit ceb7f56f4df60735615d794e0633bf2f135906cf).

Why

Part of a GetYourGuide SRE initiative to keep this repo's CLAUDE.md and .coderabbit.yaml accurate over time, using Claude (on Bedrock) to review and update them.

How it works

• Runs monthly (cron: '0 6 1 * *'), plus supports manual workflow_dispatch.
• The shared workflow only opens/updates a PR here when it detects a considerable change — it ignores routine noise like dependency bumps. Most months this should be a no-op with no visible activity in this repo.
• Uses secrets: inherit and requires contents: write, pull-requests: write, and id-token: write permissions (declared in this workflow) to open/update PRs.

Before relying on the schedule

⚠️ Please verify that the GYGROBOT_WRITE_TOKEN secret is available to this repository (org- or repo-level), since the reusable workflow depends on it to open/update PRs. If it's missing, the scheduled runs will fail silently until added.

Not merging this myself — flagging for review.