Fix agent feature gating to use resolved B2C billing entitlements

[codybmenefee]

Summary

• route Agent Dashboard access through resolved billing subscription entitlements (plan/features) in addition to session claim checks
• wire Sidebar, Settings quick-link, and Agent dashboard data hook to pass resolved plan/feature slugs into hasAgentDashboardAccess
• clarify subscribe debug logs so claim-based checks are clearly labeled as claims-only diagnostics

Why

In org-active sessions, Clerk has() claim checks can stay false for B2C user plans/features even when user billing subscription is valid. This left agent_monitor/agent_dashboard checks failing and locked Agent UI despite successful app access.

Validation

• cd app && npx eslint src/lib/agentAccess.ts src/lib/convex/useAgentDashboard.ts src/components/layout/Sidebar.tsx src/routes/app/settings.tsx src/routes/subscribe.tsx
• cd app && pnpm run build