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

coderd/database/dump.sql

@@ -1826,9 +1826,12 @@ CREATE TABLE tasks (
     template_parameters jsonb DEFAULT '{}'::jsonb NOT NULL,
     prompt text NOT NULL,
     created_at timestamp with time zone NOT NULL,
-    deleted_at timestamp with time zone
+    deleted_at timestamp with time zone,
+    display_name character varying(127) DEFAULT ''::character varying NOT NULL
 );
 
+COMMENT ON COLUMN tasks.display_name IS 'Display name is a custom, human-friendly task name.';
+
 CREATE VIEW visible_users AS
  SELECT users.id,
     users.username,
@@ -1964,6 +1967,7 @@ CREATE VIEW tasks_with_status AS
     tasks.prompt,
     tasks.created_at,
     tasks.deleted_at,
+    tasks.display_name,
         CASE
             WHEN (tasks.workspace_id IS NULL) THEN 'pending'::task_status
             WHEN (build_status.status <> 'active'::task_status) THEN build_status.status