fix: restore ctrl+c handling in gigacode (fixes #111)

[rohanranjan0902]

Description

This PR fixes an issue where the gigacode CLI does not gracefully exit when Ctrl+C is pressed.

Root Cause: The Node.js wrapper script was executing the underlying Rust binary synchronously using execFileSync. This blocked the Node event loop and prevented signals like SIGINT from properly reaching the child process.

Changes:

• Replaced execFileSync with asynchronous spawn.
• Added explicit signal forwarding for SIGINT, SIGTERM, and SIGQUIT from the parent Node process to the child Rust process.
• Ensured the parent process waits to exit until the child process completes its shutdown.

Fixes #111