docs: update CHANGELOG for 1.6.0 release 🎉

**✨ Features**
- 🌍 Add Multi-Backend Aggregation so multiple LiteLLM instances can be used concurrently and models are namespaced by backend.
- 🧭 Introduce MultiBackendClient and a new `litellm-connector.backends` setting with UI for add/edit/remove/toggle, and auto-migrate legacy single-backend config.
- 📊 Add PostHog Telemetry & Observability with typed TelemetryService, PostHog adapters, event enrichment, and aggregated reporting.
- 🧠 Add experimental V2 Chat Provider supporting VS Code's newer Language Model APIs and structured thinking/tool parts.
- ✍️ Improve multi-repo commit message generation by using SCM `rootUri`, adding `rootUri` to Repository and `findRepositoryByRootUri` in GitUtils.

**🐛 Fixes**
- 🛠️ Ensure commit-message generation failures remain visible in local logs and telemetry for better diagnosis.
- 🔧 Fix nuke command to avoid leaving stale models when resetting user configuration.
- 🧷 Stabilize telemetry ID handling and re-scope unhandled exception detection/handling to align with best practices.
- 🧭 Propagate request IDs through PostHog request telemetry and fix undefined silent flag that blocked new instance configuration.

**🧪 Testing & Validation**
- ✅ Add extensive test coverage including multi-repo regression tests, MultiBackendClient and adapter tests, PostHog Node/Web adapter tests, and model capability override tests.
- ✅ Stabilize shared test mocks and add coverage for ConfigManager feature-toggle reporting and TelemetryService feature events.

**🔧 Build & Tooling**
- 🛠️ Add packaging and tooling scripts (`package-marketplace.mjs`, `scripts/upload-posthog-sourcemaps.mjs`, `scripts/versionUtils.mjs`) and CI workflow streamlining.
- 📝 Update release header to 1.6.0 and adjust release links for the new tag.