feat(coderd): add tasks list and get endpoints (#19468)

Fixes coder/internal#899

Example API response:

```json
{
  "tasks": [
    {
      "id": "a7a27450-ca16-4553-a6c5-9d6f04808569",
      "organization_id": "241e869f-1a61-42c9-ae1e-9d46df874058",
      "owner_id": "9e9b9475-0fc0-47b2-9170-a5b7b9a075ee",
      "name": "task-hardcore-herschel-bd08",
      "template_id": "accab607-bbda-4794-89ac-da3926a8b71c",
      "workspace_id": "a7a27450-ca16-4553-a6c5-9d6f04808569",
      "initial_prompt": "What directory are you in?",
      "status": "running",
      "current_state": {
        "timestamp": "2025-08-22T10:03:27.837842Z",
        "state": "working",
        "message": "Listed root directory contents, working directory reset",
        "uri": ""
      },
      "created_at": "2025-08-22T09:21:39.697094Z",
      "updated_at": "2025-08-22T09:21:39.697094Z"
    },
    {
      "id": "50f92138-f463-4f2b-abad-1816264b065f",
      "organization_id": "241e869f-1a61-42c9-ae1e-9d46df874058",
      "owner_id": "9e9b9475-0fc0-47b2-9170-a5b7b9a075ee",
      "name": "task-musing-dewdney-f058",
      "template_id": "accab607-bbda-4794-89ac-da3926a8b71c",
      "workspace_id": "50f92138-f463-4f2b-abad-1816264b065f",
      "initial_prompt": "What is 1 + 1?",
      "status": "running",
      "current_state": {
        "timestamp": "2025-08-22T09:22:33.810707Z",
        "state": "idle",
        "message": "Completed arithmetic calculation",
        "uri": ""
      },
      "created_at": "2025-08-22T09:18:28.027378Z",
      "updated_at": "2025-08-22T09:18:28.027378Z"
    }
  ],
  "count": 2
}
```

coderd/coderd.go

@@ -1011,6 +1011,8 @@ func New(options *Options) *API {
 			r.Route("/{user}", func(r chi.Router) {
 				r.Use(httpmw.ExtractOrganizationMembersParam(options.Database, api.HTTPAuth.Authorize))
 
+				r.Get("/", api.tasksList)
+				r.Get("/{id}", api.taskGet)
 				r.Post("/", api.tasksCreate)
 			})
 		})