Prevent plugin installs from clearing unrelated plugins

[chubes4]

Summary

Remove InstallPluginStep first-ZIP-entry slug inference and manual recursive destination deletion. Plugin_Upgrader already derives and clears the package destination when overwrite_package is enabled, while also protecting the plugins root.

A valid ZIP created from the current directory may begin with ./. The old inference treated that as slug ., then deleted WP_PLUGIN_DIR/. before WordPress core could apply its protected-directory checks.

Changes

• Delegate destination selection and overwrite handling entirely to Plugin_Upgrader.
• Read the installed folder from the upgrader result for activation.
• Add a regression ZIP beginning with ./ and assert that an existing unrelated plugin survives while the new plugin installs.

Testing

• vendor/bin/phpunit components/Blueprints/Tests/Unit/Steps/InstallPluginStepTest.php (6 tests, 17 assertions)
• vendor/bin/phpunit components/Blueprints/Tests/Unit/Steps (77 tests, 184 assertions)
• vendor/bin/phpcs -d memory_limit=1G components/Blueprints/Steps/class-installpluginstep.php components/Blueprints/Tests/Unit/Steps/InstallPluginStepTest.php
• Confirmed the new regression fails before the source change and passes afterward.

Closes #310

AI assistance

OpenCode with openai/gpt-5.6-sol was used to trace the failure through PHP Toolkit and WordPress core, implement the minimal removal of redundant destination handling, and run the regression and component checks. Chris Huber reviewed and remains responsible for every line.