Update baseline build instructions to use working branch instead of main

[Copilot]

I noticed that my agents kept checking out the main branch and doing a build. Especially if/when main had diverged from whatever the working branch was based on, this was a lot of unnecessary work/time. We just want a 'before new changes' build, not main specifically.

• Update Step 2 in copilot-instructions.md to remove "checkout the main branch" and instead indicate the baseline build should run on the working branch before making changes
• Update Step 3 post-build text to remove "switch back to your working branch" language
• Run code review
• Create PR (Update baseline build instructions to use working branch instead of main #125853)

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[Copilot]

Pull request overview

Updates the baseline build guidance in .github/copilot-instructions.md to run the baseline build on the developer’s working branch (prior to making changes), rather than requiring a checkout of main, simplifying the workflow while keeping the “baseline build first” requirement.

Changes:

• Adjust Step 2 to instruct running the baseline build on the working branch before making code changes.
• Remove Step 3 language about switching back to the working branch after the baseline build.