Flatten PR graph

Put the requested PRs into a single stack in optimal merge order. Resolve merge conflicts.

Use the supplied PR list, or discover all open PRs in the current repository when the user asks for all current PRs.

1. Inspect the PRs and Git history to understand their dependencies.
2. Choose an order that respects dependencies and minimizes expected conflict-resolution work.
3. Update the branches and PR bases to form that stack, resolving conflicts while preserving the intended changes.
4. Verify the resulting chain and report the order, conflicts resolved, and anything that remains blocked or incomplete.