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/testdata/coder_task_--help.golden

@@ -0,0 +1,19 @@
+coder v0.0.0-devel
+
+USAGE:
+  coder task
+
+  Manage tasks
+
+  Aliases: tasks
+
+SUBCOMMANDS:
+    create    Create a task
+    delete    Delete tasks
+    list      List tasks
+    logs      Show a task's logs
+    send      Send input to a task
+    status    Show the status of a task.
+
+———
+Run `coder --help` for a list of global options.