chore: raise exception if user is on default branch + tries to create the PR

[christinewangcw]

Motivation

Content

Testing

Please check the following before marking your PR as ready for review

• I have added tests for my changes
• I have updated the documentation or added new documentation as needed

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
30	1	29	0

View the top 1 failed test(s) by shortest run time

::tests.unit.codegen.sdk.core.test_directory

Stack Traces | 0s run time

ImportError while importing test module '.../sdk/core/test_directory.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
.../sdk/core/test_directory.py:8: in <module>
    from codegen.sdk.codebase.codebase_graph import CodebaseGraph
E   ModuleNotFoundError: No module named 'codegen.sdk.codebase.codebase_graph'

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