Move codex module under session (#18249)

## Summary
- rename the core codex module root to session/mod.rs without using
#[path]
- move the codex module directory and tests under core/src/session
- remove session/mod.rs reexports so call sites use explicit child
module paths

## Testing
- cargo test -p codex-core --lib
- cargo check -p codex-core --tests
- just fmt
- just fix -p codex-core
- git diff --check

codex-rs/core/src/codex_thread.rs

@@ -1,8 +1,8 @@
 use crate::agent::AgentStatus;
-use crate::codex::Codex;
-use crate::codex::SteerInputError;
 use crate::config::ConstraintResult;
 use crate::file_watcher::WatchRegistration;
+use crate::session::Codex;
+use crate::session::SteerInputError;
 use codex_features::Feature;
 use codex_protocol::config_types::ApprovalsReviewer;
 use codex_protocol::config_types::Personality;