refactor(cli): drop dead c-S-c key binding (follow-up to NousResearch#19895) (NousResearch#19919)

NousResearch#19884 added a prompt_toolkit key binding for Ctrl+Shift+C to
"prevent Hermes from intercepting the keystroke as an interrupt
signal." NousResearch#19895 then wrapped the binding in try/except after
discovering it crashed startup with ValueError on every platform.

Both PRs were based on a misreading of how terminal key events
propagate:

1. Terminal emulators (GNOME Terminal, iTerm2, kitty, Windows Terminal,
   etc.) intercept Ctrl+Shift+C before the keystroke reaches the
   application's stdin. prompt_toolkit never sees it. The binding
   could never have intercepted anything.

2. prompt_toolkit's key spec parser doesn't recognise 'c-S-c' on any
   platform — the Shift modifier is meaningless on control-sequence
   keys. Verified: every prompt_toolkit version raises 'Invalid key:
   c-S-c' at registration time.

The handler is dead code. Delete it and leave a comment explaining
why no binding is needed here. Ctrl+Q alias (NousResearch#19884's other addition)
stays — that's a real prompt_toolkit key and a legitimate interrupt
shortcut.

Verified the CLI starts cleanly — key binding phase no longer raises
and the subsequent chat flow reaches the provider setup check without
error.

Author: teknium1

cli.py

@@ -10484,19 +10484,14 @@ def handle_ctrl_c(event):
                     self._should_exit = True
                     event.app.exit()
 
-        try:
-            @kb.add('c-S-c')  # Ctrl+Shift+C — no-op, let terminal handle native copy
-            def handle_ctrl_shift_c(event):
-                """No-op: let the terminal handle Ctrl+Shift+C natively.
-
-                Wrapped in try/except because prompt_toolkit raises ValueError
-                ("Invalid key: c-S-c") on platforms where this key spec is not
-                recognised.  The binding is best-effort; if registration fails,
-                startup continues normally.
-                """
-                return
-        except ValueError:
-            pass  # prompt_toolkit on this platform/version doesn't support c-S-c
+        # Ctrl+Shift+C: no binding needed. Terminal emulators (GNOME Terminal,
+        # iTerm2, kitty, Windows Terminal, etc.) intercept Ctrl+Shift+C before
+        # the keystroke reaches the application's stdin — prompt_toolkit never
+        # sees it, and prompt_toolkit's key spec parser doesn't even recognise
+        # 'c-S-c' anyway (the Shift modifier is meaningless on control-sequence
+        # keys). #19884 added a handler for this; #19895 patched the resulting
+        # startup crash with try/except. Both were based on a misreading of how
+        # terminal key events propagate. Deleting the dead handler outright.
 
         @kb.add('c-q')  # Ctrl+Q
         def handle_ctrl_q(event):