Run the tests with cgo off and without the race detector (closes #538)

sneak ruled the race detector out
(#538 (comment)), and on
Linux it was the only reason the tests needed cgo. The test phase and the
browser test binary now build with CGO_ENABLED=0 and without -race, so no
test binary compiles mattn's C SQLite. The test phase keeps the builder
stage's Debian Go image. Its comments, the README and the code comments that
said the suite runs under -race now say what is true, and the README notes
the exception to the vendored REPO_POLICIES.md.

Model: opus-5-5

Dockerfile.browser

@@ -15,9 +15,9 @@ RUN go mod download
 COPY . .
 
 # The test binary embeds the templates and static files, so the browser
-# stage needs nothing else. -p 4 keeps the compile's memory down, as in
-# the test phase of Dockerfile.
-RUN make assets && go test -c -p 4 -tags browser -o /browser.test ./internal/server
+# stage needs nothing else. It is built with cgo off and -p 4 keeps the
+# compile's memory down, both as in the test phase of Dockerfile.
+RUN make assets && CGO_ENABLED=0 go test -c -p 4 -tags browser -o /browser.test ./internal/server
 
 # chromedp/headless-shell:151.0.7922.109 (Debian trixie), 2026-08-11. The
 # browser is on PATH as headless-shell, where the test's browser library