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)

package.json

@@ -4,7 +4,8 @@
   "description": "Tools for repo-wide maintenance.",
   "scripts": {
     "format": "prettier --check *.json *.md docs/*.md .github/workflows/*.yml **/*.js",
-    "format:fix": "prettier --write *.json *.md docs/*.md .github/workflows/*.yml **/*.js"
+    "format:fix": "prettier --write *.json *.md docs/*.md .github/workflows/*.yml **/*.js",
+    "write-hooks-schema": "cargo run --manifest-path ./codex-rs/Cargo.toml -p codex-hooks --bin write_hooks_schema_fixtures"
   },
   "devDependencies": {
     "prettier": "^3.5.3"