fix: sync run button state with terminal process via centralized exit tracking

Centralize terminal process lifecycle in terminal-context as single
source of truth. TerminalView reports exit/failure via callback,
context maintains exitedTerminals set, command-dropdown reacts to it.

Removes redundant polling and per-component exit event subscriptions
that raced with deferred spawn introduced in b2d10fa.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/components/terminal/terminal-panel.tsx

@@ -5,7 +5,7 @@ import { TerminalTabBar } from "./terminal-tab-bar"
 import { TerminalView } from "./terminal-view"
 
 export function TerminalPanel() {
-  const { isOpen, tabs, activeTabId } = useTerminalContext()
+  const { isOpen, tabs, activeTabId, markTerminalExited } = useTerminalContext()
 
   return (
     <section
@@ -22,6 +22,7 @@ export function TerminalPanel() {
             initialCommand={tab.initialCommand}
             isActive={tab.id === activeTabId}
             isVisible={isOpen}
+            onProcessExited={markTerminalExited}
           />
         ))}
       </div>