Create a secret GitHub gist from the previewed file

[jongio]

Sharing a single file today means leaving grut, opening a browser, and pasting. Add a way to push the previewed file straight to a secret GitHub gist and copy the resulting URL to the clipboard.

Add a Ctrl+G keybinding in the preview that creates a secret gist from the current on-disk file using the gh CLI, then copies the gist URL and shows a toast.

Acceptance criteria:

• Ctrl+G in the preview creates a secret gist from the current file and copies its URL.
• No-op with a clear toast when the preview isn't showing an on-disk file.
• Errors from gh surface as an error toast; success shows the gist URL.
• Keybinding registered in the panel, the keybindings manifest, and the help overlay.
• Unit tests cover argument building and the non-file guard (no real gist created in tests).