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_--help.golden

@@ -53,6 +53,7 @@ SUBCOMMANDS:
     stop              Stop a workspace
     support           Commands for troubleshooting issues with a Coder
                       deployment.
+    task              Manage tasks
     templates         Manage templates
     tokens            Manage personal access tokens
     unfavorite        Remove a workspace from your favorites