feat: support startup reveal/focus by path

[Yukaii]

note: based on #2

Summary

• Add startup path resolution so bontree accepts directory, file, or explicit focus path forms
• Introduce ui.NewWithFocus to reveal/focus a target by absolute or relative path by expanding ancestor directories
• Fix initial viewport scroll behavior so focused items don't render from a cut-off top when terminal size is not known yet
• Document updated usage with optional focus-path

Testing

• go test ./ui ./config ./theme ./tree ./icons .

sample integration with kakoune

kakoune-bontree-integration.mov

[vercel]

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