Default WEBHOOKER_ENVIRONMENT to prod (closes #307)

An unset WEBHOOKER_ENVIRONMENT now means prod, not dev. The only
thing dev still changes is CORS, which then answers every origin with
Access-Control-Allow-Origin: *, so an operator who forgets the
variable is no longer silently permissive; dev must be set
explicitly. Cookie Secure and CSRF strictness follow each request's
transport and are unaffected.

The README, comments and tests no longer describe dev as the default:
the deployment checklist asks only that the environment is not dev,
the Docker and nginx examples drop the now-redundant setting, and the
TRUSTED_PROXIES warning gives its real reason for firing in every
environment.

Model: opus-4-8 (implementation); opus-5-5 (rework)

internal/session/session.go

@@ -146,8 +146,8 @@ func newStore(key []byte) *sessions.CookieStore {
 //
 // This is decided per-request, not once at startup. Deciding it at
 // startup from the configured environment is what this replaces, and
-// it got the DEFAULT posture wrong: "dev" is the environment when
-// WEBHOOKER_ENVIRONMENT is unset, so a deployment terminating TLS at a
+// it got the DEFAULT posture wrong: "dev" was then the environment when
+// WEBHOOKER_ENVIRONMENT was unset, so a deployment terminating TLS at a
 // proxy without also setting the environment emitted the
 // authentication cookie with no Secure attribute -- silently, and on
 // the same response as a CSRF cookie that did have one.