Remove image shorthand syntax {{image}} support

[codeling]

This PR removes support for the wiki-specific image shorthand syntax {{image.jpg}} in favor of standard Markdown image syntax ![Alt text](/path/to/image.jpg).

Summary

The custom image shorthand syntax has been completely removed from the codebase. This simplifies the markdown parser and reduces the attack surface for XSS vulnerabilities while maintaining full image support through standard Markdown syntax.

Key Changes

• Removed image shorthand regex pattern from functions.php that processed {{...}} syntax
• Updated documentation in MarkdownSyntax.md to remove references to the {{image.png}} shorthand syntax
• Removed UI hint from index.php drawer that suggested using {{image.jpg}} syntax
• Removed test cases for image shorthand functionality:
  • Deleted testImageShorthand() from MarkdownToHtmlTest.php
  • Deleted testImageShorthandEscapesFileName() from XssTest.php
  • Removed malicious markdown test cases for image shorthand injection attacks from MaliciousMarkdown.php
• Updated README.md to remove mention of the shorthand syntax

Implementation Details

• Users should now use standard Markdown image syntax: ![Alt text](/images/image.jpg) or ![Alt text](/images/image.jpg "Optional title")
• This change improves security by eliminating a custom parsing rule that was vulnerable to attribute injection attacks
• The removal is backward-incompatible but aligns the wiki with standard Markdown conventions

https://claude.ai/code/session_01AGfoFJAbyWfHRBNRxyhQCs