fix(isl): use correct 'remove' subcommand for worktree deletion

[sontiO]

The WorktreeRemoveOperation used 'sl wt rm' but the actual subcommand
is 'sl wt remove', causing 'abort: you need to specify a subcommand'
when attempting to delete a worktree from the UI.

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Stack created with Sapling. Best reviewed with ReviewStack.

• perf(isl): use git directly for committed diffs and file content #55
• feat(isl): show LOC stats in review mode header #54
• feat(isl): make review mode header section collapsible #53
• style(isl): unify review mode sticky header backgrounds #52
• style(isl): comprehensive UI polish across all panels #51
• style(isl): unify glassmorphism system across all sticky headers #50
• feat(isl): add PR review status indicators with legend tooltip #49
• feat(isl): show GitHub avatars for commits linked to PRs #48
• style(isl): improve PR stack layout and avatar positioning #47
• fix(isl): filter stale worktrees with non-existent directories #46
• -> fix(isl): use correct 'remove' subcommand for worktree deletion #45