Support modifying stacks across worktrees

[skarim]

Builds on #520 to let gh stack modify restructure a stack without first bringing all its branches into one worktree.

• Support rename, insert, drop, fold, and reorder across worktrees. History changes run where each affected branch is checked out; other worktrees keep their branch selections.
• Preserve dropped/folded source branches and their worktrees, while removing dropped contributions from the surviving stack, including combined drop/fold operations.
• Resume conflicts or abort from any worktree. Track action progress and expected branch tips so recovery does not replay completed steps, overwrite intervening work, or discard recovery state after a partial failure.

Only affected worktrees must be clean and available. Existing linear-stack, interactive-UI, and no-mixed-reorder/structural-action constraints remain; no automatic stashing or worktree management is added.

Completes the broader worktree-per-branch workflow discussed in #35 and Discussion #231.