Fix Shift+Enter for multiline input in agent terminals

xterm.js sends plain \r for Enter regardless of modifiers. Intercept
modified Enter in the custom key handler so it routes through CSI u
encoding (Kitty keyboard protocol), which Claude Code uses for
Shift+Enter newlines.

packages/app/src/terminal/runtime/terminal-emulator-runtime.ts

@@ -317,7 +317,9 @@ export class TerminalEmulatorRuntime {
         !shouldInterceptDomTerminalKey({
           key: normalizedKey,
           ctrlKey: event.ctrlKey,
+          shiftKey: event.shiftKey,
           altKey: event.altKey,
+          metaKey: event.metaKey,
           pendingModifiers: this.pendingModifiers,
         })
       ) {