refactor: rewrite handler-base as Go module

Replace Python handler-base library with Go module providing:
- config: environment-based configuration
- health: HTTP health/readiness server for k8s probes
- natsutil: NATS/JetStream client with msgpack serialization
- telemetry: OpenTelemetry tracing and metrics setup
- clients: HTTP clients for LLM, embeddings, reranker, STT, TTS
- handler: base Handler runner wiring NATS + health + telemetry

Implements ADR-0061 Phase 1.

.gitignore

@@ -1,23 +1,20 @@
-# Python
-__pycache__/
-*.py[cod]
-*$py.class
+# Go
+*.exe
+*.dll
 *.so
-.Python
-build/
-develop-eggs/
-dist/
-downloads/
-eggs/
-.eggs/
-lib/
-lib64/
-parts/
-sdist/
-var/
-wheels/
-*.egg-info/
-.installed.cfg
+*.dylib
+*.test
+*.out
+vendor/
+
+# IDE
+.idea/
+.vscode/
+*.swp
+
+# OS
+.DS_Store
+Thumbs.db
 *.egg
 
 # Virtual environments