Add the unauthenticated reachability probe

docs/roadmap/implementation.md

@@ -512,6 +512,13 @@ Validate: internal probe tests for 2xx/401/404/405, transport failure, timeout,
 and unsupported injected transport. Derive evidence from Stage 7 rather than
 repeat redirect matrices here. Close response bodies; do not retain them.
 
+**Completed: 2026-10-13.** Added the unauthenticated, single-GET reachability
+probe over the built-in evidence-aware HTTP client. It records any response as
+reachable regardless of status, closes and discards bodies, and reports missing
+HTTP support without fallback. Validation passed: `go test -race
+./internal/usecase ./internal/llm`, `go vet ./...`, `go test ./...`, and
+`git diff --check`.
+
 ## Stage 12: Implement Authentication Applicability And OpenRouter Probe
 
 Scope: named HTTP authentication adapter; no inference orchestration.