[codex] Fix Push MD toolkit loading and clone blockers

[artpi]

Summary

Fixes the Push MD activation and clone blockers found while testing on WordPress.com staging.

• Makes PHP Toolkit standalone function files safe to load when another plugin has already loaded the same toolkit functions.
• Updates Push MD's toolkit bootstrap paths to avoid Composer autoload-file redeclaration crashes.
• Returns Git protocol error responses for /info/refs failures instead of opaque REST 500s.
• Exports WordPress posts/pages with empty slugs using stable ID-based fallback paths, e.g. post/post-4937.md, to avoid post/.md collisions.
• Keeps those fallback paths stable while content remains slugless, without writing fallback names back into WordPress slugs; stale fallback pushes are rejected once WordPress assigns a real slug.
• Makes the Push MD bootstrap compatibility tests avoid Windows cmd.exe quoting failures.

Root Cause

WordPress.com SH (wpcomsh) already bundles wp-php-toolkit/filesystem, so Push MD's bundled toolkit could redeclare namespaced helper functions during activation. After activation was fixed, authenticated clone exposed two runtime issues: /info/refs errors were surfaced as REST 500s, and draft posts with empty slugs collided on the same export path.

The follow-up fallback-path fix avoids treating generated filenames such as post/post-4937.md as desired WordPress slugs. If WordPress later assigns a canonical slug, Push MD now asks the user to pull the slug-based path before editing rather than silently moving content back to the generated fallback path.

The Windows CI failure came from passing inline PHP snippets through php -r inside a shell command. The tests now write a temporary PHP script and invoke PHP with shell bypassing enabled on Windows.

Validation

• vendor/bin/phpunit -c phpunit.xml
• vendor/bin/phpunit -c phpunit.xml components/Filesystem/Tests/FunctionsTest.php
• vendor/bin/phpunit -c phpunit.xml --filter PMD_Bootstrap_Compatibility_Test plugins/push-md/Tests/
• vendor/bin/phpunit -c phpunit.xml plugins/push-md/Tests/
• vendor/bin/phpunit -c phpunit.xml components/Filesystem/Tests/ components/Git/Tests/ components/Zip/Tests/ components/DataLiberation/Tests/ components/Encoding/Tests/
• composer test
• php -l plugins/push-md/class-pmd-plugin.php && php -l plugins/push-md/Tests/ExportPathTest.php
• php -l plugins/push-md/Tests/BootstrapCompatibilityTest.php && php -l components/Filesystem/Tests/FunctionsTest.php
• vendor/bin/phpcs -d memory_limit=1G plugins/push-md/class-pmd-plugin.php plugins/push-md/Tests/ExportPathTest.php
• vendor/bin/phpcs -d memory_limit=1G plugins/push-md/Tests/BootstrapCompatibilityTest.php components/Filesystem/Tests/FunctionsTest.php
• git diff --check
• rebuilt dist/plugins/push-md.zip and /Users/artpi/Desktop/push-md.zip
• installed/tested on staging and confirmed git clone succeeds into ~/Desktop/site-title

Note: repo-wide composer lint still exits on pre-existing warning-level findings outside this change; scoped PHPCS for touched files passes.