feat: add display name field for tasks (#20856)

## Problem

Tasks currently only expose a machine-friendly name field (e.g.
`task-python-debug-a1b2`), but this value is primarily an identifier
rather than a clean, descriptive label. We need a separate
display-friendly name for use in the UI.

This PR introduces a new `display_name` field and updates the task-name
generation flow. The Claude system prompt was updated to return valid
JSON with both `name` and `display_name`. The name generation logic
follows a fallback chain (Anthropic > prompt sanitization > random
fallback). To make task names more closely resemble their display names,
the legacy `task-` prefix has been removed. For context, PR
https://github.com/coder/coder/pull/20834 introduced a small Task icon
to the workspace list to help identify workspaces associated to tasks.

## Changes

- Database migration: Added `display_name` column to tasks table
- Updated system prompt to generate both task name and display name as
valid JSON
- Task name generation now follows a fallback chain: Anthropic > prompt
sanitization > random fallback
- Removed `task-` prefix from task names to allow more descriptive names
- Note: PR https://github.com/coder/coder/pull/20834 adds a Task icon to
workspaces in the workspace list to distinguish task-created workspaces

**Note:** UI changes will be addressed in a follow-up PR

Related to: https://github.com/coder/coder/issues/20801

cli/exp_task_status_test.go

@@ -189,6 +189,7 @@ func Test_TaskStatus(t *testing.T) {
   "owner_id": "00000000-0000-0000-0000-000000000000",
   "owner_name": "me",
   "name": "exists",
+  "display_name": "Task exists",
   "template_id": "00000000-0000-0000-0000-000000000000",
   "template_version_id": "00000000-0000-0000-0000-000000000000",
   "template_name": "",
@@ -220,9 +221,10 @@ func Test_TaskStatus(t *testing.T) {
 					switch r.URL.Path {
 					case "/api/experimental/tasks/me/exists":
 						httpapi.Write(ctx, w, http.StatusOK, codersdk.Task{
-							ID:        uuid.MustParse("11111111-1111-1111-1111-111111111111"),
-							Name:      "exists",
-							OwnerName: "me",
+							ID:          uuid.MustParse("11111111-1111-1111-1111-111111111111"),
+							Name:        "exists",
+							DisplayName: "Task exists",
+							OwnerName:   "me",
 							WorkspaceAgentHealth: &codersdk.WorkspaceAgentHealth{
 								Healthy: true,
 							},