docs: add branching guide

[MuncleUscles]

Summary

• add docs/BRANCHING.md describing stable branches, optional integration branches, promotion PRs, E2E gates, releases, and main behavior
• replace stale contributor wording that said there is no main/default branch is stable

Testing

• Docs-only change; no test suite run.

[coderabbitai]

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/branching-guide-v040

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