gpu-tracker: remove redundant signal handler, simplify locking

[shiv-tyagi]

• Remove setupSignalHandler. flock advisory locks are kernel-managed and automatically released when the process exits, making the signal handler redundant. It also leaked a goroutine and channel on every method call. Signal handling is an application-level concern; a library should not call os.Exit or register global signal handlers, as it takes control away from the caller.
• Replace manual time.Tick poll loop in acquireLock with flock.TryLockContext, fixing a ticker leak and removing reimplemented retry logic.
• Remove defer recover from all 9 public methods. Panics indicate bugs that should crash loudly with a stack trace, not be silently converted to errors. A library should not swallow panics; whether to recover is an application-level policy that belongs in the caller, not the library.
• Simplify deferred unlock from a 4-line nil-check closure to defer lock.Unlock, since lock is guaranteed non-nil after a successful acquireLock.
• Remove redundant error wrapping on lock failures.

The signal handler, panic recovery, and verbose lock boilerplate added around 180 lines of complexity with no correctness benefit. None of these belong in a library as they override decisions that should be made by the application.

[sgopinath1]

LGTM