fix(utils): clear forceExit timer unconditionally in shutdown handler [Issue #118]

[gramseostudio-dev]

Description

This PR resolves the issue where the forceExit timer was not being cleared unconditionally during the shutdown sequence, which could lead to unexpected forceful terminations or memory leaks.

Contributor: @gramseostudio-dev

Changes Made

• Updated shutdown.ts to ensure the timeout timer is explicitly cleared upon a successful graceful shutdown.
• Updated shutdown.test.ts to verify the correct timer clearing behavior.

Testing & Verification

• Local test suite passes successfully.
• Verified graceful shutdown sequence exits cleanly without the timer firing prematurely.
• Ensured strict deliverable isolation (verified clean workspace with no agent/config files).

Closes #118