fix Ctrl-C swallowing the next line

readline only redraws the prompt on Ctrl-C: the half-typed text
stays in its buffer and the interrupt flag outlives the prompt,
so the line typed next was silently discarded. Register a signal
event hook that empties the line, the way bash does, and clear
the flag around readline instead of eating the line: a Ctrl-C
during a command still aborts the turn, and one during a
multi-line task still abandons it. Two pty-based regression
tests cover the idle-prompt and mid-line cases.

README.md

@@ -416,12 +416,13 @@ where you are when you press it:
 | -------------------- | ------------------------------------------------------------------- |
 | Running a command    | The process group is killed, exit 130 is reported, the turn ends.   |
 | Waiting on the API   | The request is abandoned and the turn ends.                         |
-| Part way through a line | The half-typed line is thrown away.                              |
-| Sitting at the prompt | Solo survives and stays usable, but the flag lingers, so the next line you type is silently discarded. |
+| Part way through a line | The half-typed line is cleared; the next line runs normally.     |
+| Mid way through a multi-line task | The whole task is abandoned.                        |
+| Sitting at the prompt | Solo survives and stays usable; the next line runs normally.      |
 
-That last case is the one wart: press Ctrl-C once at an idle prompt and your
-next command vanishes without a word. Press it twice and the second one goes
-through, so it is self-correcting in practice.
+The half-typed line is cleared by Solo itself: readline only redraws the
+prompt on Ctrl-C and leaves the text in its buffer, so Solo registers a
+signal hook that empties the line, the way bash does.
 
 ## Example session