feat: Cost alerting thresholds

[Abernaughty]

Summary

Add system-level cost alerting beyond per-task token budgets. Track daily/monthly spend across all orchestrator runs with configurable thresholds and Slack/email alerts.

Context

Per the Final Architecture (Section 6), cost alerting thresholds should include daily and monthly spend limits, per-agent cost tracking, and automatic workflow pause when budgets are exhausted.

Acceptance Criteria

• Daily and monthly spend thresholds configurable in .env or config file
• Per-agent cost tracking (Architect, Lead Dev, QA breakdown)
• Slack/email alert when thresholds are hit (webhook-based)
• Budget exhaustion auto-pauses the workflow
• Cost data persisted to local JSON or SQLite for historical tracking
• Dashboard integration (issue feat: SvelteKit Dashboard v1 (read-only + memory approvals) #17) reads this data for the Cost Tracker panel

Effort

Small (1 session)

[Abernaughty]

📋 Sprint Order: 10 of 13 (Phase 2)

⬆️ No hard blockers (dashboard #17 reads this data but isn't strictly required)
⬇️ No dependents

[Abernaughty]

Framework update: Dashboard framework switched from Streamlit to SvelteKit (see #17). Cost alerting threshold UI will be a SvelteKit route (dashboard/src/routes/costs/+page.svelte) rather than a Streamlit widget. The cost data source (Langfuse token counts + model pricing) is unchanged.