Moves shared utilities (styles, utils) from cmd/shared to pkg/cli.
Adds CLI runtime with global singleton pattern:
- cli.Init() initialises the runtime
- cli.App() returns the global instance
- OutputService for styled terminal printing
- SignalService for graceful shutdown handling
All cmd/ packages now import pkg/cli instead of cmd/shared.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
