fix: restore terminal after force closing Codex

[ducky-duke]

Summary

• capture each targeted Unix process's controlling TTY before force-killing the Codex process tree
• restore Codex's keyboard-reporting and cursor modes on that TTY after every associated target has terminated
• validate /dev paths and character devices, open with O_NOCTTY, and skip restoration when a kill partially fails
• add unit coverage for Unix ps parsing, TTY path sanitization, and target grouping

Why

Codex enables enhanced CSI-u keyboard reporting while its TUI is active. A SIGKILL bypasses normal TUI cleanup, so VS Code's terminal can remain in that mode and send encoded key sequences to the restored shell; typing and Enter then appear broken.

Validation

• cargo test --locked --manifest-path src-tauri/Cargo.toml — 34 passed in a clean Linux container with the Tauri system dependencies
• cargo fmt --manifest-path src-tauri/Cargo.toml --check
• pnpm build
• pnpm test:reset-credits — 3 passed
• git diff --check

cargo clippy --all-targets -- -D warnings could not run to completion on the host because the WebKitGTK/JavaScriptCoreGTK development packages required by Tauri are not installed. The full Rust test suite compiled and passed in the dependency-complete container.