Function format_prune_plan 

pub fn format_prune_plan(entries: &[PrunableEntry]) -> String

Render the would-do plan for gwm prune --dry-run (issue #31). Extracted from cmd_prune so the formatter is unit-testable on arbitrary PrunableEntry fixtures (non-ASCII names / paths without needing a real repo). Pure function: trailing newline included; empty input still emits the canonical “0 worktree(s) to prune” line so piped consumers get a stable signal instead of empty stdout.

Column widths are computed in Unicode characters (.chars().count()), not bytes (.len()), so non-ASCII paths stay aligned in a fixed-width terminal.