Signal() now properly forwards to cmd.Process.Signal() instead of
calling Kill(). SetDefault(nil) returns error instead of panicking.
Wait() returns descriptive errors with process context.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Process management with Core IPC integration, output streaming via
ring buffer, exec wrapper with logging, and dependency-aware runner.
Moved from forge.lthn.ai/core/go/pkg/process to standalone module.
Co-Authored-By: Virgil <virgil@lethean.io>