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>

plugin/package.json

@@ -4,7 +4,9 @@
   "private": true,
   "description": "Runtime dependencies for claude-mem bundled hooks",
   "type": "module",
-  "dependencies": {},
+  "dependencies": {
+    "@chroma-core/default-embed": "^0.1.9"
+  },
   "engines": {
     "node": ">=18.0.0",
     "bun": ">=1.0.0"