Migrate plain mode to async core

[bsg62]

Summary

• Split CLI/config/core/runner modules and move plain mode onto a shared async Engine.
• Add async health checks, process-group server spawning, output capture, final command execution, and Ctrl-C cleanup parity.
• Add AppState scaffold types for the upcoming TUI plan while keeping --tui inert.

Test Plan

• cargo test
• cargo clippy -- -D warnings
• cargo fmt -- --check
• cargo run -- --help
• cargo run -- --tui
• cargo run
• Verified port 3000 released after default run