Add module-level docs to action, record, session

[kiyeonjeon21]

Several `mod.rs` files are missing `//!` module-level documentation:

• `crates/snact-core/src/action/mod.rs`
• `crates/snact-core/src/record/mod.rs`
• `crates/snact-core/src/session/mod.rs`

These are the public API surface of snact-core. Module docs help contributors and IDE hover tooltips.

Low priority but good first issue.