Default WEBHOOKER_ENVIRONMENT to prod

An unset WEBHOOKER_ENVIRONMENT now resolves to prod rather than dev, so
an operator who forgets the variable is not silently permissive. The
only behaviour dev still changes is the CORS middleware, which answers
every origin with Access-Control-Allow-Origin: *; that is now off
unless dev is set explicitly. Cookie Secure and CSRF strictness are
decided per request from the transport and are unaffected.

Updated resolveEnvironment and its comment, the README configuration
table and prose, and tests covering the default and the explicit dev.
Comments that justified behaviour by the old dev default (the
TRUSTED_PROXIES warning, a CSRF test) were corrected; that warning
still fires in every environment when TRUSTED_PROXIES is empty.

Model: opus-4-8

internal/middleware/csrf_test.go

@@ -380,9 +380,8 @@ func csrfTookStrictPath(
 
 // TestCSRF_ForwardedProtoSpellingsTakeStrictPath runs the header
 // spellings a real proxy emits through the middleware. The environment
-// is dev -- the DEFAULT when WEBHOOKER_ENVIRONMENT is unset -- to pin
-// that the routing is a per-request transport decision and owes
-// nothing to configuration.
+// is set to dev -- the permissive setting -- to pin that the routing is
+// a per-request transport decision and owes nothing to configuration.
 func TestCSRF_ForwardedProtoSpellingsTakeStrictPath(t *testing.T) {
 	t.Parallel()