[codex] Use wp-extensions markdown editor release

[adamziel]

Stacked on #3624.

What changed

• Replace the local sqlite-markdown extension compile project with the Markdown Editor runtime release published from adamziel/wp-extensions.
• Add playground-cli:download-edit-markdown-runtime to download markdown-editor-latest/wp-markdown-editor.zip during CLI builds.
• Mount the released markdown-editor directory as wp-content/mu-plugins and load its sqlite_markdown PHP 8.4 manifest.
• Remove the php-toolkit submodule from Playground and ignore the generated release cache.

Notes

• The current wp-extensions release only ships a PHP 8.4 sqlite_markdown artifact, so edit-markdown now defaults to PHP 8.4 and rejects other PHP versions.

Testing

• npx nx run playground-cli:download-edit-markdown-runtime
• npx nx run playground-cli:typecheck
• npx nx run playground-cli:build
• npx nx run playground-cli:lint
• git submodule update --init isomorphic-git
• npx nx run playground-cli:test-playground-cli