feat: Add DefaultEmbeddingFunction for local vector embeddings

- Added @chroma-core/default-embed dependency for local embeddings
- Updated ChromaSync to use DefaultEmbeddingFunction with collections
- Added isServerReachable() async method for reliable server detection
- Fixed start() to detect and reuse existing Chroma servers
- Updated build script to externalize native ONNX binaries
- Added runtime dependency to plugin/package.json

The embedding function uses all-MiniLM-L6-v2 model locally via ONNX,
eliminating need for external embedding API calls.

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

package.json

@@ -86,6 +86,7 @@
   },
   "dependencies": {
     "@anthropic-ai/claude-agent-sdk": "^0.1.76",
+    "@chroma-core/default-embed": "^0.1.9",
     "@modelcontextprotocol/sdk": "^1.25.1",
     "ansi-to-html": "^0.7.2",
     "chromadb": "^3.2.2",