Add Windows Terminal pane splits for opencode

[ndycode]

Summary

• add Windows Terminal pane splits for opencode 2/4/6 (or --panes)
• keep pane layout stable for 2x2 and 3x2
• fix Windows path handling (patch parsing, worktree, ignore rules, plugin resolve, bash paths)
• stabilize Windows tests (path normalization, POSIX-only skips, long filename guard)
• fix custom-elements reference files for app/enterprise on Windows

Testing

• bun test
• bun turbo typecheck --filter=@opencode-ai/app --filter=@opencode-ai/desktop
• bun turbo typecheck --filter=@opencode-ai/enterprise