Drop the unused project description field entirely

It never got a UI home on the frontend -- removed from ProjectView a
few sessions back and never restored anywhere -- so it was a fully
live field (validated, stored, returned by the API, covered by tests)
with no consumer.

- Migration 011: ALTER TABLE projects DROP COLUMN description.
- ProjectRepository: description dropped from ProjectRow, SELECT,
  create() and update() -- update() is now just a rename (string, not
  a $fields array; there was never more than one editable field once
  this left).
- ProjectController: store()/update() no longer accept or validate
  it; update() drops the "provide at least one of title, description"
  branch, since title is unconditionally the only field now.
- Frontend Project type, root README's API docs and curl example.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/ProjectTest.php

@@ -15,15 +15,11 @@ final class ProjectTest extends ApiTestCase
     {
         $auth = $this->authHeader();
 
-        $created = $this->request('POST', '/api/projects', [
-            'title' => '  Groceries  ',
-            'description' => 'Weekly shop',
-        ], $auth);
+        $created = $this->request('POST', '/api/projects', ['title' => '  Groceries  '], $auth);
 
         self::assertSame(201, $created->getStatusCode());
         $project = $this->decode($created)['project'];
         self::assertSame('Groceries', $project['title']);
-        self::assertSame('Weekly shop', $project['description']);
         self::assertSame(0, $project['card_count']);
 
         $index = $this->decode($this->request('GET', '/api/projects', null, $auth));
@@ -63,7 +59,7 @@ final class ProjectTest extends ApiTestCase
 
     public function test_project_creation_validates_title(): void
     {
-        $response = $this->request('POST', '/api/projects', ['description' => 'no title'], $this->authHeader());
+        $response = $this->request('POST', '/api/projects', [], $this->authHeader());
 
         self::assertSame(422, $response->getStatusCode());
         self::assertArrayHasKey('title', $this->decode($response)['error']['details']);