Link between articles with [[Article name]]

[bonelifer]

Adds links between wiki articles, as asked for in the extension's phpBB.com topic.

• Syntax: [[Article name]] links to that article; [[Article name|other text]] shows different text. It works in wiki articles and in forum posts. Brackets inside [code] stay plain text.
• Missing articles: links to articles that don't exist yet are shown in red with a tooltip, and following one opens the editor to create the article.
• What links here: each article lists the articles whose current version links to it.
• Editor button: a "Wiki link" button in the editor toolbar inserts the brackets, with a tooltip and screen-reader text.
• Older text: gets its links when next edited, or all at once with php bin/phpbbcli.php reparser:reparse phpbbmodders_wiki_article (and post_text for posts). The README documents this, with three new screenshots.

@ was avoided on purpose: phpBB 4.0 uses @name to mention members.

How it works: the text formatter turns the markup into a WIKILINK tag that renders without an address; after rendering, the extension adds each link's address from the wiki's own route and marks the articles that don't exist. "What links here" is read from the stored article text, so there is no new table or migration.

Testing
Claude added a phpbbmodders/wiki check to phpbbmodders/phpbb-release-tools' feature_checks.py (branch wiki-feature-check): all 8 checks pass on this branch and the link checks fail on main, as they should. The smoke test is clean, the new PHP passes php -l on PHP 7.4, template_a11y.py reports nothing, and Claude checked that the reparser converts older articles and posts.

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Investigated and written by Claude on behalf of William Jacoby (bonelifer).