gh: support bk PR build failure detective

[v1v]

What is the problem this PR solves?

Use Agentic Workflows to troubleshoot any BK failures

See elastic.github.io/ai-github-actions/workflows/gh-agent-workflows/estc-pr-buildkite-detective

How does this PR solve the problem?

Help our teams to be more efficient when debugging errors in the CI.

How to test this PR locally

CI only

Design Checklist

• I have ensured my design is stateless and will work when multiple fleet-server instances are behind a load balancer.
• I have or intend to scale test my changes, ensuring it will work reliably with 100K+ agents connected.
• I have included fail safe mechanisms to limit the load on fleet-server: rate limiting, circuit breakers, caching, load shedding, etc.

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool

Related issues

See elastic/elastic-agent#14004