refactor(agent): consolidate agent system into unified AgentManager with provider-based architecture

Replace separate ACP and agent-types modules with unified AgentManager using provider pattern. Migrate Claude and Codex implementations to provider-specific clients. Streamline session-agent integration, remove old ACP notification types, and introduce agent timeline events. Delete obsolete documentation and test files. Add AgentRegistry for persistence and agent-stream-based communication pattern.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

packages/app/src/components/mode-selector-modal.tsx

@@ -46,7 +46,7 @@ export function ModeSelectorModal({
                     isActive && styles.modeNameActive,
                   ]}
                 >
-                  {mode.name}
+                  {mode.label}
                 </Text>
                 {mode.description && (
                   <Text