Improve agent display names in MessageInput

Update display names to be more concise:
- "opencode" → "OpenCode" (consistent capitalization)
- "Codex CLI (ACP)" → "Codex" (simplified, keep "(ACP)" in settings only)

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/main/config/defaults.ts

@@ -15,14 +15,14 @@ export const DEFAULT_AGENTS: Record<string, AgentConfig> = {
   },
   opencode: {
     id: 'opencode',
-    name: 'opencode',
+    name: 'OpenCode',
     command: 'opencode',
     args: ['acp'],
     enabled: true
   },
   codex: {
     id: 'codex',
-    name: 'Codex CLI (ACP)',
+    name: 'Codex',
     command: 'codex-acp',
     args: [],
     enabled: true