refactor: centralize types and extract seed data (Task 1.1)

- Create types/ directory with centralized domain types:
  session, tool-instance, tool-type, git-repository, config-folder,
  tool-config, project, user, api-response
- Remove inline type definitions from API modules;
  re-export from types/ for backward compatibility
- Update state/sessions.tsx to import Session from types/session.ts
- Update all consumer components/pages to import from types/
- Extract seed_builtin_tool_types from main.py to
  seeds/builtin_tool_types.py
- Create types/index.ts barrel export

Quality gates: tsc (pass), eslint (pass), Python syntax (pass)

apps/web/src/components/instance-list.tsx

@@ -1,7 +1,8 @@
 import { useCallback, useEffect, useState } from "react";
 import { useNavigate } from "react-router-dom";
 import { Icon } from "./icon";
-import type { ToolInstance } from "../api/sessions";
+import type { ToolInstance } from "../types/tool-instance";
+import type { ToolType } from "../types/tool-type";
 import {
   checkInstanceHealth,
   createInstance,
@@ -12,7 +13,6 @@ import {
   startInstance,
   stopInstance,
 } from "../api/sessions";
-import type { ToolType } from "../api/tool_types";
 
 const API_BASE_URL = import.meta.env.VITE_API_BASE_URL ?? "http://localhost:8000";