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/pages/sessions.tsx

@@ -2,20 +2,22 @@ import { useCallback, useEffect, useMemo, useState } from "react";
 import { useNavigate } from "react-router-dom";
 
 import { listProjects } from "../api/projects";
-import type { Project } from "../types";
-import { listRepositories, type GitRepository } from "../api/git_repositories";
+import { listRepositories } from "../api/git_repositories";
 import {
   getUserSessions,
-  type Session,
   deleteInstance,
   stopInstance,
   startInstance,
   checkInstanceHealth,
   recreateInstanceTunnel,
+  createInstance,
 } from "../api/sessions";
-import { listToolTypes, type ToolType } from "../api/tool_types";
-import { createInstance } from "../api/sessions";
+import { listToolTypes } from "../api/tool_types";
 import { getUserConfig, updateUserConfig } from "../api/settings";
+import type { Project } from "../types/project";
+import type { Session } from "../types/session";
+import type { GitRepository } from "../types/git-repository";
+import type { ToolType } from "../types/tool-type";
 import { Icon } from "../components/icon";
 
 type SessionsStatus = "loading" | "ready" | "error";