Expose executor environment snapshot

[caomengxuan666]

Changes

• Add a public Executor::env_vars_snapshot accessor mirroring aliases_snapshot.
• Allows host shells to synchronize shell-local environment state after source/interactive execution without exposing mutable executor internals.

Validation

• cargo test --manifest-path C:/Users/Administrator/rubash/Cargo.toml env_vars_snapshot --lib
  • Builds successfully; filter has no direct test match, so 0 tests run.