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/skills.rs

@@ -3,9 +3,9 @@ use std::collections::HashSet;
 use std::env;
 use std::sync::Arc;
 
-use crate::codex::Session;
-use crate::codex::TurnContext;
 use crate::config::Config;
+use crate::session::session::Session;
+use crate::session::turn_context::TurnContext;
 use codex_analytics::InvocationType;
 use codex_analytics::SkillInvocation;
 use codex_analytics::build_track_events_context;