refactor: split context modules

[beinan]

Summary

• re-export context types from the crate root after splitting modules (rust/lance-context/src/lib.rs:3)
• keep the in-memory context API in its own module for clarity (rust/lance-context/src/context.rs:3)
• isolate record structs and Lance-backed store into dedicated files without changing behavior (rust/lance-context/src/record.rs:3, rust/lance-context/src/store.rs:18)
• update the CI helper skill to invoke gh pr create when available (.codex/skills/ci-pr-helper/SKILL.md:11)

Testing

• ./.codex/skills/ci-pr-helper/scripts/run_ci_checks.sh (fails: uv not found)
• PATH=$HOME/.cargo/bin:$PATH cargo fmt --manifest-path rust/lance-context/Cargo.toml