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/models.go

@@ -4218,6 +4218,7 @@ type Task struct {
 	Prompt                       string                           `db:"prompt" json:"prompt"`
 	CreatedAt                    time.Time                        `db:"created_at" json:"created_at"`
 	DeletedAt                    sql.NullTime                     `db:"deleted_at" json:"deleted_at"`
+	DisplayName                  string                           `db:"display_name" json:"display_name"`
 	Status                       TaskStatus                       `db:"status" json:"status"`
 	StatusDebug                  json.RawMessage                  `db:"status_debug" json:"status_debug"`
 	WorkspaceBuildNumber         sql.NullInt32                    `db:"workspace_build_number" json:"workspace_build_number"`
@@ -4241,6 +4242,8 @@ type TaskTable struct {
 	Prompt             string          `db:"prompt" json:"prompt"`
 	CreatedAt          time.Time       `db:"created_at" json:"created_at"`
 	DeletedAt          sql.NullTime    `db:"deleted_at" json:"deleted_at"`
+	// Display name is a custom, human-friendly task name.
+	DisplayName string `db:"display_name" json:"display_name"`
 }
 
 type TaskWorkspaceApp struct {