Add branch summary documentation

[google-labs-jules]

This change adds a new file BRANCHES.md which contains a summary of all branches in the repository. The summary includes the branch name, inferred purpose, last update date, and the author of the last update. This was generated by analyzing the git branch list and commit history.

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PR created automatically by Jules for task 11443805616129802715 started by @ithinkihaveacat

[google-labs-jules]

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[kul3r4]

@ithinkihaveacat ok to close this PR?