Resolve stale forge.lthn.ai/core/cli v0.1.0 references (tag never existed,
earliest is v0.0.1) and regenerate go.sum via workspace-aware go mod tidy.
Co-Authored-By: Virgil <virgil@lethean.io>
Replace the Phase 3 stub with a working ProxyProvider that reverse-proxies
requests to upstream provider binaries via httputil.ReverseProxy. Implements
Provider + Renderable interfaces. Includes 9 tests covering proxy routing,
health passthrough, element spec, and invalid upstream handling.
Co-Authored-By: Virgil <virgil@lethean.io>