Add per-environment DAG branch config

[thomasyu888]

Problem:

• The DAG git-sync branch is hardcoded to main in values.yaml, meaning dev and prod always sync DAGs from the same branch with no way to test DAG changes in isolation

Solution:

• Added a dags_git_branch variable threaded through the Spacelift stack modules and the Airflow module, overriding dags.gitSync.branch and dags.gitSync.ref via ArgoCD Helm parameters
• Dev environment syncs DAGs from orca-recipes:develop, prod from orca-recipes:main
• Staging falls back to the default of main

Testing:

• Confirm dev Airflow is syncing DAGs from the develop branch of orca-recipes
• Confirm prod Airflow continues to sync from main

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