docs(iteration-41): define dev branch workflow

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Summary

• Record that Iteration 41 changes across backtrader and the seven bt_api repositories are committed separately and reviewed into each repository's dev branch.
• Set each dev branch as the baseline for subsequent Iteration 41 development; pause new feature work until current changes are inventoried and submitted.

Checks

• git diff --check
• Verified the branch-governance link target exists.