docs(e2e): rewrite e2e README for the shared cassettes and cleanup

Refocus the README on how frodo-cli uses frodo-lib's record/replay
support and point to the new frodo-lib developer guide for the mechanics.
Update it for the shared login cassettes for cloud, forgeops and classic,
connection types and FRODO_MOCK_DEPLOYMENT, replay-time profile
isolation, and running with the right frodo binary. Add a section on
finding and removing orphaned recordings, note that recordings must not
carry a positional host, and record the known test problems found while
cleaning up.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PqcSs7nnL9SCPfu5aY38ns

Author: vscheuber