Move extension registration into extension.json

[alistair3149]

Moves the extension's registration out of the procedural root SemanticMetaTags.php entry point into native extension.json declarations, so the extension no longer keeps a root PHP file or a callback.

Changes

• Settings → config. The five smtg* settings defaults move from DefaultSettings.php into the extension.json config section with config_prefix: smtg. The global names (smtgTagsProperties, smtgTagsStrings, smtgTagsBlacklist, smtgTagsPropertyFallbackUsage, smtgMetaPropertyPrefixes) and their default values are unchanged, so existing LocalSettings.php overrides keep working. DefaultSettings.php is deleted. Using config rather than a callback matters: a callback runs after LocalSettings.php and would overwrite user overrides, whereas config values are true defaults the user wins over.
• Extension function → namespaced class. onExtensionFunction moves into a new SMT\Setup class, and ExtensionFunctions points at SMT\Setup::onExtensionFunction. The two runtime hooks it builds (OutputPageParserOutput, BeforePageDisplay) need the store and options, so they stay registered from the extension function.
• Removed dead/redundant code. Drops the callback, AutoloadClasses, the SMT_VERSION constant, the never-wired doCheckRequirements() method, and the !defined( 'SMW_VERSION' ) die guard (unreachable now that requires.extensions.SemanticMediaWiki gates loading). The message directory is already declared via MessagesDirs.
• Tests. tests/bootstrap.php reads the version from extension.json instead of the removed SMT_VERSION constant. I18nJsonFileIntegrityTest is guarded against wgMessagesDirs now resolving to the manifest array rather than the string the removed procedural assignment used to set.
• Deletes the root SemanticMetaTags.php.

Verification

• composer analyze clean (lint + PHPCS + minus-x) and composer phpunit green (80 tests) in the docker-compose-ci stack on MediaWiki 1.43 + SMW dev-master.
• Live page-view smoke test: with a static-string tag and a property-backed tag configured, both <meta> elements are injected into the real page HTML, confirming the migrated extension function and hook registration drive the render path end to end.
• Confirmed via eval.php that the config_prefix defaults populate the smtg* globals, the runtime hooks register, the old SemanticMetaTags class is gone, and SMT\Setup is present.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.88%. Comparing base (2eecb7d) to head (ace2aa8).

Files with missing lines	Patch %	Lines
src/Setup.php	0.00%	12 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #113      +/-   ##
============================================
- Coverage     82.43%   77.88%   -4.56%     
- Complexity       80       81       +1     
============================================
  Files             7        8       +1     
  Lines           205      217      +12     
============================================
  Hits            169      169              
- Misses           36       48      +12     

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