fix(cli): allow multiple depends-on args in coder exp sync want (#23869)

Previously the command required exactly two arguments, forcing users to
run it multiple times to declare multiple dependencies for a single
unit.
This accepts variadic depends-on arguments so all dependencies can be
declared in one call:

```
coder exp sync want my-unit dep-1 dep-2 dep-3
```

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: Marcin Tojek <mtojek@users.noreply.github.com>

cli/testdata/coder_exp_sync_--help.golden

@@ -16,7 +16,7 @@ SUBCOMMANDS:
     ping        Test agent socket connectivity and health
     start       Wait until all unit dependencies are satisfied
     status      Show unit status and dependency state
-    want        Declare that a unit depends on another unit completing before it
+    want        Declare that a unit depends on other units completing before it
                 can start
 
 OPTIONS: