start of hooks engine (#13276)

(Experimental)

This PR adds a first MVP for hooks, with SessionStart and Stop

The core design is:

- hooks live in a dedicated engine under codex-rs/hooks
- each hook type has its own event-specific file
- hook execution is synchronous and blocks normal turn progression while
running
- matching hooks run in parallel, then their results are aggregated into
a normalized HookRunSummary

On the AppServer side, hooks are exposed as operational metadata rather
than transcript-native items:

- new live notifications: hook/started, hook/completed
- persisted/replayed hook results live on Turn.hookRuns
- we intentionally did not add hook-specific ThreadItem variants

Hooks messages are not persisted, they remain ephemeral. The context
changes they add are (they get appended to the user's prompt)

justfile

@@ -82,6 +82,10 @@ write-config-schema:
 write-app-server-schema *args:
     cargo run -p codex-app-server-protocol --bin write_schema_fixtures -- "$@"
 
+[no-cd]
+write-hooks-schema:
+    cargo run --manifest-path ./codex-rs/Cargo.toml -p codex-hooks --bin write_hooks_schema_fixtures
+
 # Tail logs from the state SQLite database
 log *args:
     if [ "${1:-}" = "--" ]; then shift; fi; cargo run -p codex-state --bin logs_client -- "$@"