refactor: drop the ignored secure toggle parameter from session.NewManager

Per review on PR #48: pre-1.0 there is no installed base to keep
compiling against, so remove the dead parameter in one pass instead of
deferring. NewManager now takes only the signing key. Update the
handlers.go call site and its stale comment, mechanically update the
NewManager call shapes in the session tests (assertions unchanged),
collapse the now-meaningless constructor-argument loop in the cookie
attributes test, and drop the moot P1 Future Step from TODO.md.

Reviewer-directed test call-site updates; cookie behavior is unchanged
from the previous commit (always Secure/HttpOnly/SameSite=Strict).

internal/session/session.go

@@ -44,10 +44,8 @@ type Manager struct {
 // Session cookies always carry the Secure, HttpOnly, and SameSite=Strict
 // attributes; this cannot be configured. Browsers treat http://localhost as a
 // trustworthy origin and accept Secure cookies there, so local development
-// keeps working. The second parameter is the former secure toggle: it is
-// ignored and retained only so existing call sites keep compiling; it will be
-// removed in a follow-up change.
-func NewManager(signingKey string, _ bool) (*Manager, error) {
+// keeps working.
+func NewManager(signingKey string) (*Manager, error) {
 	masterKey := []byte(signingKey)
 
 	// Derive separate keys for HMAC (hash) and encryption (block)