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).

TODO.md

@@ -56,9 +56,6 @@ returns 410; logout redirects back to login
 - P0: implement cache size management and eviction so the disk cannot
   fill up
 - P0: validate configuration on startup, fail fast on bad config
-- P1: remove the ignored former secure toggle parameter from
-  session.NewManager and update its call sites (requires touching
-  existing tests; needs approval per repo rules)
 - P1: implement blocked networks configuration to extend SSRF
   protection
 - P1: rate limit global concurrent upstream fetches to prevent