Stop describing dev as the default environment

Comments in reqtls and session, and the README Quick Start and
deployment checklist, still spoke as if an unset WEBHOOKER_ENVIRONMENT
meant dev. The checklist now asks only that the environment is not dev
(unset or prod); the Quick Start describes running from the clone.

Model: opus-5-5

internal/session/session_test.go

@@ -990,8 +990,8 @@ func sessionCookieFrom(
 
 // TestSave_SecureFollowsRequestTransport is the regression test for
 // the defect this replaces: Secure was fixed at startup from the
-// configured environment, and "dev" is the environment when
-// WEBHOOKER_ENVIRONMENT is unset. A deployment behind a TLS proxy in
+// configured environment, and "dev" was then the environment when
+// WEBHOOKER_ENVIRONMENT was unset. A deployment behind a TLS proxy in
 // that DEFAULT posture shipped the authentication cookie with no
 // Secure attribute and said nothing about it.
 //