Move fixture apps from twenty-sdk to twenty-apps/fixtures (#18531)

## Summary

- Move 4 test fixture apps from `twenty-sdk/src/cli/__tests__/apps/` to
`twenty-apps/fixtures/` with meaningful names (`rich-app` →
`postcard-app`, `root-app` → `minimal-app`)
- Replace all `from '@/sdk'` imports with `from 'twenty-sdk'` so fixture
apps are proper, portable twenty-sdk apps
- Remove the fragile `"@/*": ["../../../../../src/*"]` tsconfig hack and
replace with standard `"src/*": ["./src/*"]` paths
- Create a centralized `fixture-paths.ts` utility in twenty-sdk tests
for clean app path resolution

## Why

The fixture apps were deeply nested in twenty-sdk's test directory and
tightly coupled to its internal source layout via a tsconfig path alias
hack. This made them:
- Impossible to reuse outside of SDK CLI tests (e.g., for server-side
dev seeding with `DevSeederService`)
- Fragile — moving any twenty-sdk source file could break the path alias
- Poorly discoverable — buried 5 directories deep in test infrastructure

Moving them to `twenty-apps/fixtures/` makes them first-class portable
apps that can be imported by `twenty-server` for seeding, used in E2E
testing, and serve as canonical examples alongside `hello-world`.

## Test plan

- [x] All 8 twenty-sdk integration tests pass (3 suites: postcard-app,
minimal-app, invalid-app)
- [x] Prettier formatting verified on all changed files
- [ ] CI should confirm E2E tests also pass (these require a running
server)

Made with [Cursor](https://cursor.com)

packages/twenty-apps/fixtures/function-execute-app/src/logic-functions/add-numbers.function.ts

@@ -0,0 +1,24 @@
+import { defineLogicFunction } from 'twenty-sdk';
+
+export const ADD_NUMBERS_UNIVERSAL_IDENTIFIER =
+  'f9e5589c-e951-4d99-85db-0a305ab53502';
+
+const addNumbersHandler = (payload: { a: number; b: number }) => {
+  const result = payload.a + payload.b;
+
+  console.log(`Adding ${payload.a} + ${payload.b} = ${result}`);
+
+  return result;
+};
+
+export default defineLogicFunction({
+  universalIdentifier: ADD_NUMBERS_UNIVERSAL_IDENTIFIER,
+  name: 'add-numbers',
+  timeoutSeconds: 5,
+  handler: addNumbersHandler,
+  httpRouteTriggerSettings: {
+    path: '/add-numbers',
+    httpMethod: 'GET',
+    isAuthRequired: false,
+  },
+});