Update root and per-directory AGENTS.md files to reflect the current
state of the codebase: collection inventory, RBAC registry, base
management, personnel pages, training minigames, Discord bot transfer
and evaluation-reminder flows, banking currency config, migration
wrapper, and Playwright/Vivaldi e2e details.
Add domain-specific AGENTS.md files for collections, components, lib,
utils, and bot. Add login-return-url case study documenting the
return-URL flow design decisions and debugging lessons learned.
