Wire CostTracker to agent loop

[yacosta738]

Context

Parent: DALLAY-164 (Cost Governance Productization)
Proposal: openspec/changes/2026-04-06-cost-governance-productization/proposal.md

Problem

CostTracker in src/cost/tracker.rs is fully implemented with tests but never instantiated in production — it's dead code. No provider TokenUsage is piped into it, and no pre-flight budget check gates LLM calls.

Goal

Activate cost tracking and budget enforcement in the agent loop, behind the existing cost.enabled feature flag (defaults to false).

Acceptance Criteria

• CostTracker::new() is instantiated during runtime bootstrap when cost.enabled = true
• Provider responses pipe TokenUsage into CostTracker::record_usage() after each LLM call
• CostTracker::check_budget() is called as a pre-flight gate before each LLM call
• When budget is exceeded (BudgetCheck::Exceeded), the LLM call is rejected with a structured error
• When budget is at warning threshold (BudgetCheck::Warning), a warning is emitted via observer
• All enforcement is gated behind cost.enabled flag — zero behavior change when disabled
• JSONL storage rotation/pruning strategy is defined (at minimum, configurable retention window)
• Providers that don't return token counts use estimation fallback from CostConfig.prices

Technical Notes

• CostTracker uses in-memory cache with JSONL flush — check_budget() is O(1)
• CostConfig already has daily_limit_usd, monthly_limit_usd, warn_at_percent, prices
• This is the foundation — all other cost governance issues (B-F) depend on this

[linear]

DALLAY-255