[Console] Deprecate custom-rule command

[TomasVotruba]

The custom-rule command assumed too much about a project's layout (composer autoload-dev keys, phpunit test suite wiring, PHPUnit attribute vs annotation detection). Scaffolding a local custom rule is now handled more reliably by AI agents.

execute() no longer generates files - it prints an error and returns exit code 1:

 [ERROR] The "custom-rule" command is deprecated and no longer generates
         files. Use an AI agent to scaffold a custom rule instead - it
         handles the setup faster and with less guesswork.

Also removed:

• templates/custom-rule and templates/custom-rules-annotations
• now-unused JsonFileSystem::writeFile() and ClassName::DATA_PROVIDER
• stale ext-dom composer suggest that referenced the command

https://claude.ai/code/session_01RiHnNDzk3DFHHgXojtfjoz