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)

codex-rs/hooks/src/bin/write_hooks_schema_fixtures.rs

@@ -0,0 +1,9 @@
+use std::path::PathBuf;
+
+fn main() -> anyhow::Result<()> {
+    let schema_root = std::env::args_os()
+        .nth(1)
+        .map(PathBuf::from)
+        .unwrap_or_else(|| PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("schema"));
+    codex_hooks::write_schema_fixtures(&schema_root)
+}