fix: align PHP RPC namespaces and response handler

[tolga-tom-nook]

Summary

• aligns Composer package metadata and PSR-4 autoloading with the repository's naftalimurgor\Bitgesell namespaces
• moves Config out of the stale Denpa\Bitcoin namespace and updates the client config provider
• fixes the response handler/class mismatch (BitgeselldResponse typo vs BitcoindResponse file/tests)
• replaces the stale duplicated Go README with PHP Bitgesell Core RPC usage notes

Validation

• python3 -m json.tool composer.json
• static scan confirmed no remaining Denpa\Bitcoin, BitgeselldResponse, denpa/php-bitcoinrpc, or stale Go README references
• git diff --check

Notes

PHP/Composer are not installed in this runner, so runtime PHPUnit execution was not available here. The changed symbols are direct namespace/class-string mismatches that would otherwise break Composer autoloading and response wrapping.