refactor: consolidate boolean params into options structs

[boneskull]

Per PR #25 review feedback, doCascadeWithState and doSubmitPushAndPR have accumulated multiple boolean parameters that are easy to misuse at call sites.

Suggested refactoring:

• Create CascadeOptions struct with UpdateOnly, Web, PushOnly, AllBranches, StashRef fields
• Create SubmitOptions struct with DryRun, UpdateOnly, OpenWeb, PushOnly fields
• Update function signatures and all call sites

This makes calls self-documenting and prevents future flag additions from requiring churn across the codebase.