feat(cli): promote tasks commands from experimental to GA (#20916)

## Overview

This change promotes the tasks CLI commands from `coder exp task` to
`coder task`, marking them as generally available (GA).

## Migration

Users will need to update their scripts from:

```shell
coder exp task create "my task"
```

To:
```shell
coder task create "my task"
```

---

🤖 This change was written by Claude Sonnet 4.5 Thinking using [mux](https://github.com/coder/mux) and reviewed by a human 🏄🏻‍♂️

cli/task.go

@@ -0,0 +1,25 @@
+package cli
+
+import (
+	"github.com/coder/serpent"
+)
+
+func (r *RootCmd) tasksCommand() *serpent.Command {
+	cmd := &serpent.Command{
+		Use:     "task",
+		Aliases: []string{"tasks"},
+		Short:   "Manage tasks",
+		Handler: func(i *serpent.Invocation) error {
+			return i.Command.HelpHandler(i)
+		},
+		Children: []*serpent.Command{
+			r.taskCreate(),
+			r.taskDelete(),
+			r.taskList(),
+			r.taskLogs(),
+			r.taskSend(),
+			r.taskStatus(),
+		},
+	}
+	return cmd
+}