[shell-operator] feat: add options to shell constructor

[ldmonster]

Overview

Summary

This is a major architectural refactoring of the shell-operator that introduces:

🔧 Core Architecture Changes

1. Flexible Constructor Pattern

• NEW: Options-based constructor with NewShellOperatorWithOptions()
• BENEFIT: More testable, maintainable, and extensible initialization

2. Centralized Metrics Management

• NEW: Comprehensive metrics package at metrics.go
• MOVED: All metric constants from scattered files into single location
• ADDED: Background metric updaters for live ticks and queue lengths

3. Task System Refactoring

• MOVED: TaskResult → task.Result (moved to domain level)
• NEW: Interface definitions for TaskQueue and TaskQueueSet
• IMPROVED: Better encapsulation with private fields and getter methods
• OPTIMIZED: Enhanced task queue compaction with object pooling

🔄 API Evolution

Old API:

// Rigid initialization
operator, err := shell_operator.Init(logger)

New API:

// Flexible, configurable initialization
operator, err := shell_operator.NewShellOperatorWithOptions(ctx,
    config.WithLogger(logger),
    config.WithMetricStorage(storage),
    config.WithHookMetricStorage(hookStorage),
)

What this PR does / why we need it

Special notes for your reviewer