Seed CHANGELOG.md and SETUP.md in create-twenty-app scaffold (#22769)

Projects scaffolded with `create-twenty-app` now include two additional
seed files:

- `CHANGELOG.md` with an initial `0.1.0` entry matching the template's
package version
- `SETUP.md` with step-by-step local setup instructions (prerequisites,
install, local server, dev sync, verification commands)

Both files live in `src/constants/template/`, so they flow through the
existing `fs.copy` scaffolding and the vite `copy-assets` build step
with no code changes. Verified `dist/constants/template/` contains both
files after `nx build create-twenty-app`.

The scaffolded `README.md` was also simplified into a marketable front
page for the app being built: a pitch placeholder, a features section,
and links to `SETUP.md` for setup instructions and `CHANGELOG.md` for
history, instead of duplicating dev commands.

Also:

- Adds a regression test asserting the template directory contains both
seed files
- Updates `project-structure.mdx` docs to list the new files in the
scaffold directory tree

---------

Co-authored-by: Martin <martin@twenty.com>

packages/twenty-docs/developers/extend/apps/getting-started/project-structure.mdx

@@ -33,7 +33,7 @@ my-twenty-app/
   vitest.unit.config.ts                     # Unit test runner config
   tsconfig.json, tsconfig.spec.json
   .nvmrc, .yarnrc.yml, .oxlintrc.json
-  README.md, AGENTS.md, CLAUDE.md
+  README.md, AGENTS.md, CLAUDE.md, CHANGELOG.md, SETUP.md
 ```
 
 ## Key files
@@ -47,6 +47,7 @@ my-twenty-app/
 | `src/__tests__/` | A unit test plus an integration test (with its global setup) that syncs the app against a real server. |
 | `public/` | Static assets (images, fonts) served with your app. |
 | `AGENTS.md` / `CLAUDE.md` | Guidance for AI coding agents working on the app. |
+| `CHANGELOG.md` / `SETUP.md` | Changelog of notable changes and setup instructions for local development. |
 
 <Note>
 **File organization is up to you.** The folders above are conventions — the SDK detects entities via AST analysis on `export default defineEntity(...)` calls regardless of where the file lives.