fix(ui/agents): drop Codex-incompatible --model example from custom args tab

The helper text and placeholder suggested `--model claude-sonnet-4-…` as
a custom CLI argument, which is valid for Claude but crashes Codex
agents (its `app-server` subcommand does not accept model flags). Swap
in provider-agnostic copy so the UI no longer steers users into an
invalid configuration for non-Claude runtimes.

Refs #1308.

packages/views/agents/components/tabs/custom-args-tab.tsx

@@ -77,8 +77,8 @@ export function CustomArgsTab({
             Custom Arguments
           </Label>
           <p className="text-xs text-muted-foreground mt-0.5">
-            Additional CLI arguments appended to the agent command at launch
-            (e.g. --model claude-sonnet-4-20250514)
+            Additional CLI arguments appended to the agent command at launch.
+            Supported flags depend on the agent's CLI.
           </p>
         </div>
         <Button
@@ -99,7 +99,7 @@ export function CustomArgsTab({
               <Input
                 value={entry.value}
                 onChange={(e) => updateEntry(index, e.target.value)}
-                placeholder="--model claude-sonnet-4-20250514"
+                placeholder="--flag value"
                 className="flex-1 font-mono text-xs"
               />
               <button