Move codex module under session

[pakrym-oai]

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