feat(cli): replace open vscode container with devcontainer subagent (#18765)

This change allows a devcontainer to be opened via the agent syntax,
`coder open vscode <workspace>.<agent>` and removes the `--container`
option to simplify the subcommand. Accessing the subagent will behave
similarly to how the `--container` option behaved.

Fixes coder/internal#748

cli/speedtest.go

@@ -83,7 +83,7 @@ func (r *RootCmd) speedtest() *serpent.Command {
 				return xerrors.Errorf("--direct (-d) is incompatible with --%s", varDisableDirect)
 			}
 
-			_, workspaceAgent, err := getWorkspaceAndAgent(ctx, inv, client, false, inv.Args[0])
+			_, workspaceAgent, _, err := getWorkspaceAndAgent(ctx, inv, client, false, inv.Args[0])
 			if err != nil {
 				return err
 			}