fix(clients): keep the automatic prompt read off herdr's harvesting path

Both direct-mode clients read a freshly blocked pane with `--source recent` and
no `--format`, which is herdr's harvesting read: asked for more rows than the
pane shows, it walks the agent's own scroll interface to fetch the rest, moving
the operator's terminal to do it. The relay has always used `visible` here for
exactly that reason (PROMPT_READ_SOURCE); the clients never followed.

Verified on a 48-row idle claude pane: `--lines 200 --source recent` returns
101 rows of genuine older output beyond the viewport -- `visible` is exactly its
tail -- and the CLI default format is the harvesting one, since no `--format`
and `--format text` agree at 137 non-blank rows while `--format ansi` returns
the 37 on screen. Windows asks for 50 rows, so that pane is already past its
viewport today; macOS asks for 20, inside most viewports but not a pane split
down small.

Nothing is given up: a prompt is on screen by definition. The Windows helper
serves a user-initiated read too, where reaching past the viewport is the point,
so the source became a parameter defaulting to `recent` -- its one other caller
(RelayConnection.cs:386) passes two positional arguments and is unchanged.