feat(embeddings): Add pluggable embedding provider system for AgenticDB

Implements a proper embedding abstraction layer to replace the hash-based placeholder:

## New Features

### EmbeddingProvider Trait
- Pluggable interface for any embedding system
- Methods: embed(), dimensions(), name()
- Thread-safe (Send + Sync)

### Built-in Providers
- **HashEmbedding**: Original placeholder (default, backward compatible)
- **ApiEmbedding**: Production-ready API providers (OpenAI, Cohere, Voyage AI)
- **CandleEmbedding**: Stub for candle-transformers (feature: real-embeddings)

### AgenticDB Updates
- New constructor: `AgenticDB::with_embedding_provider(options, provider)`
- Backward compatible: `AgenticDB::new(options)` still works with HashEmbedding
- Dimension validation ensures provider matches database configuration

### Files Added
- src/embeddings.rs: Core embedding provider system
- tests/embeddings_test.rs: Comprehensive test suite
- docs/EMBEDDINGS.md: Complete usage documentation
- examples/embeddings_example.rs: Working example

### Usage
```rust
// Production (OpenAI)
let provider = Arc::new(ApiEmbedding::openai(&key, "text-embedding-3-small"));
let db = AgenticDB::with_embedding_provider(options, provider)?;
```

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: ruvnet

Cargo.lock

@@ -43,6 +43,9 @@ once_cell = { workspace = true }
 chrono = { workspace = true }
 uuid = { workspace = true, features = ["v4"] }
 
+# HTTP client for API embeddings
+reqwest = { version = "0.11", default-features = false, features = ["blocking", "json", "rustls-tls"] }
+
 [dev-dependencies]
 criterion = { workspace = true }
 proptest = { workspace = true }
@@ -81,6 +84,7 @@ storage = ["redb", "memmap2"]  # File-based storage (not available in WASM)
 hnsw = ["hnsw_rs"]  # HNSW indexing (not available in WASM due to mmap dependency)
 memory-only = []  # Pure in-memory storage for WASM
 uuid-support = []  # Deprecated: uuid is now always included
+real-embeddings = []  # Feature flag for embedding provider API (use ApiEmbedding for production)
 
 [lib]
 crate-type = ["rlib"]