[Refactor] Extract TaskStackManager from ClineProvider.ts #22

Parent Issue

Part of #8 — Code Refactor on Monolith Files

Summary

Extract the task stack (LIFO) management and task event forwarding from src/core/webview/ClineProvider.ts into a dedicated TaskStackManager module.

Scope

Methods to extract:

• addClineToStack()
• removeClineFromStack()
• performPreparationTasks()
• getTaskStackSize()
• getCurrentTaskStack()
• getCurrentTask()
• Task event listener setup and forwarding logic

State owned:

• clineStack: Task[]
• taskEventListeners: WeakMap<Task, Array<() => void>>

Estimated size: ~400–450 lines

Notes

This is a pure orchestration layer — it bridges Task lifecycle events to Provider-level events. Extracting it removes ~430 lines from ClineProvider.ts and gives delegation/subtask management a clear home.

Constraints (from #8)

• Preserve event emission and listener teardown order
• Avoid circular dependencies between TaskStackManager and Task
• Run type checks after extraction
• One extraction per commit