docs: reorganize CONTRIBUTING.md local build section

[anandgupta42]

What does this PR do?

Reorganizes the local build section in CONTRIBUTING.md for clarity:

• Moves "Project structure" above "Building a local binary" so readers know the layout first
• Consolidates all commands under a single cd packages/opencode block
• Mentions Rosetta 2 handling

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Issue for this PR

Closes #264

How did you verify your code works?

Documentation-only change. Verified markdown renders correctly.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• New and existing tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works

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