A small include to use in testing SearchBoxArgs

(:title text:)
Sets a page's title to be something other than the page's name. The title text can contain apostrophes and other special characters. If there are multiple titles in a page, the last one encountered wins (see also $EnablePageTitlePriority about how to change it).
(:toc:), (:notoc:)
Override the default table of contents settings. This directive requires a corresponding configuration of the wiki, see Table of contents Basic automatic table of contents and numbered headings

Sets a page's summary. Used in some of the default PageList Templates. Note, this is a PageTextVariable and unlike core page directives it is case sensitive.

Can I get (:redirect:) to return a "moved permanently" (HTTP 301) status code?

Use (:redirect PageName status=301:).

Is there any way to prevent the "redirected from" message from showing at the top of the target page when I use (:redirect:)?

From version 2.2.1 on, set in config.php $EnableRedirectQuiet=1; and in the page (:redirect OtherPage quiet=1:) for a quiet redirect.

Is there any method for redirecting to the equivalent page in a different group, i.e. from BadGroup?/thispage => GoodGroup?/thispage using similar markup to (:redirect Goodgroup.{Name}:)?

(:redirect Goodgroup.{$Name}:) works if you want to put it in one page.
If you want it to work for the entire group, put (:redirect Goodgroup.{*$Name}:) into Badgroup.GroupHeader? - however, that only works with pages that really exist in Goodgroup; if you visit a page in Badgroup without a corresponding page of the same name in Goodgroup, instead of being redirected to a nonexistant page, you get the redirect Directive at the top of the page.
With (:if exists Goodgroup.{*$Name}:)(:redirect Goodgroup.{*$Name}:)(:ifend:) in Badgroup.GroupHeader? you get redirected to Goodgroup.Name if it exists, otherwise you get Badgroup.Name without the bit of code displayed.

How can a wiki enable linebreaks by default, i.e. without having the directive (:linebreaks:) in a page or in a GroupHeader?

Add to config.php such a line:
$HTMLPNewline = '<br/>';

 0: 00.00 00.00 config start
 1: 00.01 00.01 config end
 2: 00.22 00.20 MarkupToHTML begin
 3: 00.26 00.24 MarkupToHTML end
 4: 00.26 00.24 MarkupToHTML begin
 5: 00.27 00.25 ReadApprovedUrls SiteAdmin.ApprovedUrls begin
 6: 00.27 00.25 ReadApprovedUrls SiteAdmin.ApprovedUrls end
 7: 00.28 00.26 MarkupToHTML end
 8: 00.28 00.26 MarkupToHTML begin
 9: 00.28 00.26 MarkupToHTML end
10: 00.28 00.26 now
Peak memory: 3,912,072 bytes