feat(selector): add split-pane file preview to interactive picker

- Add @inquirer/core for custom prompt creation
- Create file-selector.ts with split-pane layout (file list + preview)
- Show full path with ~ for home above preview pane
- Add fuzzy filtering with character highlighting
- Support PgUp/PgDn for preview scrolling
- Cache file contents to avoid repeated disk reads

Also fixes: preserve ~/.mdflow in PATH when changing directories

Author: johnlindquist