Enforce kelos-worker PRs to include proper closing issue reference

Problem

The /squash-commits workflow requires PRs to have a closing issue reference (e.g., Fixes #123) that the extractClosingIssueNumbers regex can parse. However, kelos-worker sometimes writes PR descriptions where the issue number is embedded in natural language rather than as a proper closing keyword reference.

For example, PR #494 had:

Fixes the uninstall ordering to properly handle CRDs, custom resources, and finalizers (#461)

The regex expects Fixes #461 directly, but the #461 was separated from the keyword by other text. This caused /squash-commits to fail with:

PR #494 does not declare a fixed issue in this repository

Expected Behavior

kelos-worker should always include a proper closing issue reference in PR descriptions, formatted as:

Fixes #<issue_number>

This should appear as a standalone line or clearly parseable reference so that GitHub's auto-close and our workflow regex can detect it.

Acceptance Criteria

• kelos-worker PR descriptions always contain a Fixes #<issue_number> reference that matches the standard GitHub closing keyword format
• The /squash-commits workflow can reliably extract the issue number from kelos-worker PRs

Start with the /squash-commits workflow and its extractClosingIssueNumbers regex to understand the required closing-reference format. Then trace where kelos-worker builds PR descriptions; done means generated descriptions include a parseable standalone Fixes #<issue_number> reference and the workflow extracts it successfully.