nook: add /agents, /changelog and /faq

Three organic surfaces, each answering an objection at the moment it
bites. /agents lists all twelve agents with each hook's exact event
surface (mirrors AgentSpec.all + hook/setup.go + the plugins) and what
a card can do — allow/deny or observe. /changelog renders the appcast
HTML the Sparkle updater already publishes, so release notes have one
source of truth. /faq answers privacy (scoped honestly: no prompt or
code content leaves the device), the notch question, battery,
permissions, the macOS 14 floor and offline use — no refund question,
per decision: the trial is the answer. Nav gains the three links.

Tests: appcast parser suite (5 tests) covers CDATA, escaped HTML,
Version-prefix titles and garbage input.

Issues: FRE-12, FRE-13, FRE-14

src/lib/nav-config.ts

@@ -120,6 +120,9 @@ export const NAV_CONFIG: Record<SiteId, NavItem[]> = {
   ],
   nook: [
     { label: "Home", href: "/", icon: "home" },
+    { label: "Agents", href: "/agents", icon: "home" },
+    { label: "Changelog", href: "/changelog", icon: "home" },
+    { label: "FAQ", href: "/faq", icon: "home" },
     { label: "Privacy", href: "/privacy", icon: "privacy" }
   ]
 };