Sync slice/internal → main

[github-actions]

Automated PR – triggered by commit 0a415ba

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PR-Codex overview

This PR updates the GitHub Actions workflow for syncing changes from the slice/internal branch to the main branch instead of the master branch.

Detailed summary

• Changed the base branch from master to main in the PR existence check.
• Updated the title of the PR creation command from "Sync slice/internal → master" to "Sync slice/internal → main".

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