Cost optimization recommendations and budget gates

## Description

The cost estimator skill estimates monthly costs but offers no optimization recommendations or budget thresholds. The manifesto emphasizes cost as a compliance dimension enforced across generation, plan, and deploy phases.

## Scope

1. **Budget thresholds** — Configurable monthly budget in `.azure/git-ape.yml`. Deployments exceeding the budget require explicit override.
2. **Cost optimization recommendations** — After estimation, suggest reserved instances, right-sizing, savings plans.
3. **Azure Cost Management integration** — Query Azure Advisor for cost optimization recommendations on deployed resources.
4. **Cost trend tracking** — Track cost estimates across deployments over time in `state.json`.
5. **PR cost comparison** — When a template changes, show cost delta in the plan PR comment.

## Acceptance Criteria

- [ ] Budget threshold is configurable.
- [ ] Deployments exceeding budget are flagged/blocked.
- [ ] Cost optimization suggestions included in estimates.
- [ ] Cost deltas shown when templates are modified.
- [ ] Historical cost tracking in deployment state.