feat: add deleteBy, batched auto-save, source roundtrip

[danielebarbaro]

Summary

Implements deleteBy(sourceType, ?sourceName) and a few supporting changes, as discussed on ezimuel/PHPVector#7.

PHPVector exposes the primitives (metadataSearch() from #4, deleteDocument() from #3), so this PR composes them.
No neuron-core/neuron-ai dependency is added to PHPVector.

Changes

• sourceType / sourceName round-trip: folded into PHPVector metadata as reserved keys on write; restored and stripped from metadata on read in similaritySearch. Enables deleteBy to filter on them.
• deleteBy(sourceType, ?sourceName): builds an ANDed MetadataFilter list, runs metadataSearch, deletes each match via deleteDocument, then persist().
• deleteBySource(...) (deprecated by the interface) becomes a one-line delegate.
• Configurable batched auto-save: new bool \$autoSave = true constructor flag; persist() calls save() once per public mutation, gated by \$autoSave && \$database->isPersistent() so in-memory databases are skipped. The pre-existing manual-save path is kept on a single test.
• README: usage, persistence semantics (answering the load/save question from #7), autoSave, deletion.

Dependency

Requires ezimuel/phpvector ^0.3.0 (now stable on Packagist).

[ilvalerione]

Thank you Daniele for the contribution.

I thought to simplify the usage of this component managing the creation of a new database or loading an exsting one automatically inside the PHPVector class.

So now the adapter just get the path of the directory to store database information instead of forcing developers to implement the check logic:

class MyRAG extends RAG
{
    ...

    protected function vectorStore(): VectorStoreInterface
    {
        return new PHPVector(
            path: '/var/data/mydb',
            topK: 5,
        );
    }
}

Here is the code: https://github.com/neuron-core/php-vector/blob/main/src/PHPVector.php#L33-L41

Let me know what you think.