✨ feat: add basic-cli exit effect

[zrr1999]

Summary

• add Exit to the basic-cli platform handles and expose basic_cli.cmd.exit(code: Int) -> Never uses [Exit]
• extend the host crate dispatch surface with host_exit coverage and tests while keeping it explicit that the live runtime still lives in spore
• document the new basic_cli.cmd capability surface in the README and reject unsupported exit codes instead of truncating them silently

Validation

• cargo test --manifest-path host/Cargo.toml
• ../spore/target/debug/spore check src/basic_cli/cmd.sp
• ../spore/target/debug/spore build src/basic_cli/cmd.sp

Notes

• this PR only adds the package surface and host-side mirror; the live project-mode runtime plumbing lands separately in spore
• the current exit-code boundary is 0..=255, matching the CLI-side process boundary used by the paired spore runtime slice