Development record

The brief at .abcd/development/brief/04-surfaces/11-history.md:192-195 and :203 says both staging entrypoints exit 0 on every path, and that on SubagentStop every failure path is a diagnostic and an exit 0. The registered hook commands, hooks/session-end.sh and hooks/subagent-stop.sh, exit 1 when the plugin binary is missing or a legacy ~/.abcd stands; only `abcd hook session-end` and `abcd hook subagent-stop` themselves always exit 0.