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/root.go

@@ -104,6 +104,7 @@ func (r *RootCmd) CoreSubcommands() []*serpent.Command {
 		r.resetPassword(),
 		r.sharing(),
 		r.state(),
+		r.tasksCommand(),
 		r.templates(),
 		r.tokens(),
 		r.users(),
@@ -149,7 +150,6 @@ func (r *RootCmd) AGPLExperimental() []*serpent.Command {
 		r.mcpCommand(),
 		r.promptExample(),
 		r.rptyCommand(),
-		r.tasksCommand(),
 		r.boundary(),
 	}
 }