fix(agents): terminal fills the panel on maximize

- Mount xterm as an absolute inset-0 fill so it always has an explicit box and
  fit() measures the real size when the dock expands (was collapsing via the
  flex/percentage height chain).
- Acquire the pty slave as the child's controlling terminal (setsid + TIOCSCTTY)
  so the kernel delivers SIGWINCH on a browser resize. Previously the initial
  size was read once but live resizes were dropped, so the pane stayed at its
  startup size — content never grew with the window.

app/src/components/AgentTerminal.tsx

@@ -219,13 +219,18 @@ export function AgentTerminal({
           </button>
         )}
       </div>
+      {/* Sized region + an absolutely-filled xterm mount. The absolute inset-0
+          child always has an explicit box equal to its parent, so `fit()` never
+          measures a stale/percentage-collapsed height — the terminal tracks the
+          panel as it expands. */}
       <div
-        ref={containerRef}
-        className={cn(
-          maximized || fill ? "flex-1 min-h-0 p-2" : "h-[560px] p-2",
-          compact && "max-sm:p-0",
-        )}
-      />
+        className={cn("relative", maximized || fill ? "flex-1 min-h-0" : "h-[560px]")}
+      >
+        <div
+          ref={containerRef}
+          className={cn("absolute inset-0 p-2", compact && "max-sm:p-0")}
+        />
+      </div>
     </div>
   )
 }