core: remove cross-crate re-exports from lib.rs (#16512)

## Why

`codex-core` was re-exporting APIs owned by sibling `codex-*` crates,
which made downstream crates depend on `codex-core` as a proxy module
instead of the actual owner crate.

Removing those forwards makes crate boundaries explicit and lets leaf
crates drop unnecessary `codex-core` dependencies. In this PR, this
reduces the dependency on `codex-core` to `codex-login` in the following
files:

```
codex-rs/backend-client/Cargo.toml
codex-rs/mcp-server/tests/common/Cargo.toml
```

## What

- Remove `codex-rs/core/src/lib.rs` re-exports for symbols owned by
`codex-login`, `codex-mcp`, `codex-rollout`, `codex-analytics`,
`codex-protocol`, `codex-shell-command`, `codex-sandboxing`,
`codex-tools`, and `codex-utils-path`.
- Delete the `default_client` forwarding shim in `codex-rs/core`.
- Update in-crate and downstream callsites to import directly from the
owning `codex-*` crate.
- Add direct Cargo dependencies where callsites now target the owner
crate, and remove `codex-core` from `codex-rs/backend-client`.

codex-rs/core/src/hook_runtime.rs

@@ -16,6 +16,7 @@ use codex_protocol::protocol::AskForApproval;
 use codex_protocol::protocol::EventMsg;
 use codex_protocol::protocol::HookCompletedEvent;
 use codex_protocol::protocol::HookRunSummary;
+use codex_protocol::protocol::HookStartedEvent;
 use codex_protocol::user_input::UserInput;
 use serde_json::Value;
 
@@ -306,7 +307,7 @@ async fn emit_hook_started_events(
     for run in preview_runs {
         sess.send_event(
             turn_context,
-            EventMsg::HookStarted(crate::protocol::HookStartedEvent {
+            EventMsg::HookStarted(HookStartedEvent {
                 turn_id: Some(turn_context.sub_id.clone()),
                 run,
             }),