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_send_--help.golden

@@ -0,0 +1,21 @@
+coder v0.0.0-devel
+
+USAGE:
+  coder task send [flags] <task> [<input> | --stdin]
+
+  Send input to a task
+
+    - Send direct input to a task.:
+  
+       $ coder task send task1 "Please also add unit tests"
+  
+    - Send input from stdin to a task.:
+  
+       $ echo "Please also add unit tests" | coder task send task1 --stdin
+
+OPTIONS:
+      --stdin bool
+          Reads the input from stdin.
+
+———
+Run `coder --help` for a list of global options.