Open a file at path and line from the command line

[jongio]

Let grut accept a file path, optionally with a line number, so you can jump straight into a file. Today grut [dir] only takes a directory. Accepting a file makes grut usable as the target of shell pipelines and quick jumps like grut internal/git/client.go:120.

Solution

When the positional argument points at a file, open grut rooted at the file's directory, select that file in the File Tree, focus the Preview panel, and scroll to the line when a :line suffix is present. Keep the existing directory behavior unchanged.

Acceptance criteria

• grut path/to/file opens with that file selected and shown in the Preview panel.
• grut path/to/file:42 additionally scrolls the preview to line 42.
• A trailing :line on a path that does not exist is treated as part of the directory argument, matching current behavior.
• Passing a directory continues to work exactly as before.
• Unit tests cover the file-vs-directory argument parsing and the :line suffix handling.