[Refactor] Extract token & tool usage tracking into UsageTracker from Task.ts #18

Parent Issue

Part of #8 — Code Refactor on Monolith Files

Summary

Extract the token and tool usage accumulation cluster from src/core/task/Task.ts into a dedicated UsageTracker module.

Scope

Methods to extract:

• getTokenUsage()
• recordToolUsage()
• recordToolError()
• emitFinalTokenUsageUpdate()

State owned:

• tokenUsageSnapshot?
• tokenUsageSnapshotAt?
• toolUsageSnapshot?: ToolUsage
• debouncedEmitTokenUsage (debounced throttle function)
• TOKEN_USAGE_EMIT_INTERVAL_MS = 2000

Estimated size: ~150–200 lines

Notes

Simple accumulator pattern with a clear boundary. No circular dependency risk. Good candidate for unit testing once isolated.

Constraints (from #8)

• Preserve method names and call signatures
• Run type checks after extraction
• One extraction per commit