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).