[codex] Remove legacy after tool use hooks (#21805)

## Why

The legacy `AfterToolUse` hook path was still wired through core tool
dispatch even though the hooks registry never populated any handlers for
it. The supported hook surface is `PostToolUse`, so the old
infrastructure was dead code on the hot path.

## What changed

- Removed the legacy `AfterToolUse` dispatch from `codex-core` tool
execution.
- Removed the unused legacy hook payload types and exports from
`codex-hooks`.
- Simplified legacy notify handling now that `HookEvent` only carries
`AfterAgent`.

## Validation

- `cargo test -p codex-hooks`
- `cargo test -p codex-core registry`

codex-rs/hooks/src/lib.rs

@@ -69,13 +69,9 @@ pub use schema::write_schema_fixtures;
 pub use types::Hook;
 pub use types::HookEvent;
 pub use types::HookEventAfterAgent;
-pub use types::HookEventAfterToolUse;
 pub use types::HookPayload;
 pub use types::HookResponse;
 pub use types::HookResult;
-pub use types::HookToolInput;
-pub use types::HookToolInputLocalShell;
-pub use types::HookToolKind;
 
 /// Returns the hook event label used in persisted hook-state keys.
 pub fn hook_event_key_label(event_name: HookEventName) -> &'static str {