Fix estc-pr-buildkite-detective default docs

> [!CAUTION]
> Protected files were modified in this change.
> This pull request is in `request_review` mode and requires explicit human scrutiny before merge.
>
> Protected files: `README.md`

## Summary
- align the documented default for `report-failure-as-issue` with actual workflow behavior
- update `gh-agent-workflows/estc-pr-buildkite-detective/README.md` default from `true` to `false`
- update `docs/workflows/gh-agent-workflows/estc-pr-buildkite-detective.md` default from `true` to `false`

## Why
Issue #2070 reported a behavior/docs mismatch: the source workflow default is `false`, but both docs locations said `true`.

Closes \#2070.

---
[What is this?](https://ela.st/github-ai-tools) | [From workflow: Trigger Bug Hunter](https://github.com/elastic/ai-github-actions/actions/runs/34593221319)

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---

> [!NOTE]
> This was originally intended as a pull request, but the git push operation failed.
>
> **Original error:** ERR_VALIDATION: pushSignedCommits: refusing unsigned push for branch 'fix-2070-estc-buildkite-doc-default-1b7851c3fa56ea36': Signed-commit payload violates file-protection policy (request_review): gh-agent-workflows/estc-pr-buildkite-detective/README.md
>
> **Workflow Run:** [View run details and download bundle artifact](https://github.com/elastic/ai-github-actions/actions/runs/34593221319)
>
> The bundle file is available in the `agent` artifact in the workflow run linked above.

To create a pull request with the changes:

```sh
# Download the artifact from the workflow run
gh run download 34593221319 -n agent -D /tmp/agent-34593221319

# Fetch the bundle into a temporary ref, then update the local branch
git fetch /tmp/agent-34593221319/aw-fix-2070-estc-buildkite-doc-default.bundle refs/heads/fix-2070-estc-buildkite-doc-default:refs/bundles/create-pr-fix-2070-estc-buildkite-doc-default-1b7851c3fa56ea36-d1627cf9
git update-ref refs/heads/fix-2070-estc-buildkite-doc-default-1b7851c3fa56ea36 refs/bundles/create-pr-fix-2070-estc-buildkite-doc-default-1b7851c3fa56ea36-d1627cf9
git checkout fix-2070-estc-buildkite-doc-default-1b7851c3fa56ea36
# Ensure the working tree matches the updated branch
git reset --hard
# Remove the temporary bundle ref
git update-ref -d refs/bundles/create-pr-fix-2070-estc-buildkite-doc-default-1b7851c3fa56ea36-d1627cf9

# Push the branch to origin
git push origin fix-2070-estc-buildkite-doc-default-1b7851c3fa56ea36

# Create the pull request
gh pr create --title 'Fix estc-pr-buildkite-detective default docs' --base main --head fix-2070-estc-buildkite-doc-default-1b7851c3fa56ea36 --repo elastic/ai-github-actions
```