refactor: consolidate duplicate AgentConfig interface

Moved the AgentConfig interface definition from SettingsModal.tsx and
NewInstanceModal.tsx to the shared types file (src/renderer/types/index.ts)
to eliminate code duplication and establish a single source of truth.

Changes:
- Added AgentConfig interface to src/renderer/types/index.ts
- Updated SettingsModal.tsx to import AgentConfig from shared types
- Updated NewInstanceModal.tsx to import AgentConfig from shared types

Resolves housekeeping task #6

src/renderer/components/NewInstanceModal.tsx

@@ -1,12 +1,6 @@
 import React, { useState, useEffect } from 'react';
 import { Folder, X } from 'lucide-react';
-
-interface AgentConfig {
-  id: string;
-  name: string;
-  available: boolean;
-  path?: string;
-}
+import type { AgentConfig } from '../types';
 
 interface NewInstanceModalProps {
   isOpen: boolean;

src/renderer/components/SettingsModal.tsx

@@ -1,12 +1,6 @@
 import React, { useState, useEffect, useMemo } from 'react';
 import { X, Key, Moon, Sun, Keyboard, Check } from 'lucide-react';
-
-interface AgentConfig {
-  id: string;
-  name: string;
-  available: boolean;
-  path?: string;
-}
+import type { AgentConfig } from '../types';
 
 interface SettingsModalProps {
   isOpen: boolean;

src/renderer/types/index.ts

@@ -102,3 +102,10 @@ export interface Group {
   emoji: string;
   collapsed: boolean;
 }
+
+export interface AgentConfig {
+  id: string;
+  name: string;
+  available: boolean;
+  path?: string;
+}