Combine update_readme job with create_exercise job in start-exercise workflow

[Copilot]

Changes

Merged update_readme job into create_exercise to reduce workflow latency and complexity.

Key changes:

• Moved README update steps into create_exercise job after issue creation
• Added repository checkout step at job start
• Changed reference from needs.create_exercise.outputs.issue-url to steps.create-issue.outputs.ISSUE_URL
• Removed standalone update_readme job and its dependency chain

Result: Single job executes both operations sequentially without runner spin-up overhead. Job outputs remain unchanged.

Checklist

• I have added or updated appropriate labels to this PR
• I have tested my changes
• I have updated the documentation if needed

Original prompt

In start-exercise workflow combine the update_readme job with the create_exercise job

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