[johnbillion]

When pluginFile isn't set, wp-compat looks for {directory name}.php. This fails in git worktrees and any other checkout whose directory isn't named after the plugin, and PHPStan reports it as an internal error with a stack trace and a request to file a PHPStan bug.

This PR:

• Falls back to scanning the top-level PHP files for a Plugin Name header, as WordPress' get_plugins() does.
• Replaces the "No plugin or theme file found" exception with a message that says which config parameters to set.
• Gives a specific message when the configured pluginFile doesn't exist.

Written with AI assistance (Claude Code).

Note: composer test:phpstan reports assign.overwritten for $methodName in processMethodCall(). That's existing code and is not changed here.