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/memories/phase1.rs

@@ -1,7 +1,5 @@
 use crate::Prompt;
 use crate::RolloutRecorder;
-use crate::codex::Session;
-use crate::codex::TurnContext;
 use crate::config::Config;
 use crate::contextual_user_message::is_memory_excluded_contextual_user_fragment;
 use crate::memories::metrics;
@@ -10,6 +8,8 @@ use crate::memories::phase_one::PRUNE_BATCH_SIZE;
 use crate::memories::prompts::build_stage_one_input_message;
 use crate::rollout::INTERACTIVE_SESSION_SOURCES;
 use crate::rollout::policy::should_persist_response_item_for_memories;
+use crate::session::session::Session;
+use crate::session::turn_context::TurnContext;
 use codex_api::ResponseEvent;
 use codex_config::types::MemoriesConfig;
 use codex_otel::SessionTelemetry;

codex-rs/core/src/memories/phase2.rs

@@ -1,7 +1,5 @@
 use crate::agent::AgentStatus;
 use crate::agent::status::is_final as is_final_agent_status;
-use crate::codex::Session;
-use crate::codex::emit_subagent_session_started;
 use crate::config::Config;
 use crate::memories::extensions::PendingExtensionResourceRemoval;
 use crate::memories::extensions::find_old_extension_resources;
@@ -13,6 +11,8 @@ use crate::memories::prompts::build_consolidation_prompt;
 use crate::memories::storage::rebuild_raw_memories_file_from_memories;
 use crate::memories::storage::rollout_summary_file_stem;
 use crate::memories::storage::sync_rollout_summaries_from_memories;
+use crate::session::emit_subagent_session_started;
+use crate::session::session::Session;
 use codex_config::Constrained;
 use codex_features::Feature;
 use codex_protocol::ThreadId;

codex-rs/core/src/memories/start.rs

@@ -1,7 +1,7 @@
-use crate::codex::Session;
 use crate::config::Config;
 use crate::memories::phase1;
 use crate::memories::phase2;
+use crate::session::session::Session;
 use codex_features::Feature;
 use codex_protocol::protocol::SessionSource;
 use std::sync::Arc;

codex-rs/core/src/memories/tests.rs

@@ -416,14 +416,14 @@ task_outcome: success
 mod phase2 {
     use crate::ThreadManager;
     use crate::agent::AgentControl;
-    use crate::codex::Session;
-    use crate::codex::make_session_and_context;
     use crate::config::Config;
     use crate::config::test_config;
     use crate::memories::memory_root;
     use crate::memories::phase2;
     use crate::memories::raw_memories_file;
     use crate::memories::rollout_summaries_dir;
+    use crate::session::session::Session;
+    use crate::session::tests::make_session_and_context;
     use chrono::Duration as ChronoDuration;
     use chrono::Utc;
     use codex_config::Constrained;