Centralize Go validation and build environments

docs/roadmap/implementation.md

@@ -1,6 +1,6 @@
 # ReleaseKit audit implementation plan
 
-Status: Stage 1 complete; Stage 2 not started.
+Status: Stages 1 and 2 complete; Stage 3 not started.
 Baseline: `e4be7fa` (2026-10-06). The [audit](audit.md) records the evidence,
 product goals, and selected direction. This document owns implementation order
 and completion criteria for that work.
@@ -160,6 +160,14 @@ survive. Preserve existing version injection, empty-target, cancellation, and
 partial-build behavior. Fixtures use temporary Go configuration and never edit
 the developer's persistent settings.
 
+**Completed:** Shared Go configuration now drives default validation,
+toolchain/host queries, target builds, and the native smoke build. Failing
+validation fixtures prove process and persistent `GOFLAGS` cannot suppress a
+real test and that cross-target settings still run tests on the host. Build
+metadata confirms ambient and persistent tuning are cleared, and a temporary
+persistent cache is used. Focused tests, the full offline suite, API boundary
+checks, and documentation whitespace checks pass.
+
 ## Stage 3 — Make product checks additive
 
 **Covers:** F4 and ReleaseKit's own F1 exposure. **Primary owners:** `check.go`,