chore: update tagline to "Awesomely fast virtual sandbox with bash and file system"

[chaliy]

Summary

• Updates the project tagline from "Virtual bash interpreter for multi-tenant environments" to "Awesomely fast virtual sandbox with bash and file system"
• Changed in all 4 locations: root Cargo.toml (comment + description field), README.md, and crates/bashkit/src/lib.rs rustdoc

Test plan

• cargo test --all-features — all 116 tests pass
• cargo fmt --check — clean
• cargo clippy --all-targets --all-features -- -D warnings — clean
• No code logic changed — metadata/comments only