feat: implement burnless diff command

[Dinushi94]

Closes #3

Summary

Two modes in one command:

Mode 1 — no args (preview what apply would create)

burnless diff
Loaded: sre.yaml
Service: payments-api

Would create:
  + SLO: availability (target: 99.9%, window: 30d)
  + 2 Prometheus alerting rule(s)
  + 2 runbook(s) (1 auto, 1 semi-auto)
  + 1 Grafana dashboard (4 panel(s)) via grafana
  + 1 on-call policy via pagerduty (escalation: 10 min)

Run 'burnless apply' to deploy these changes.

Mode 2 — two files (compare configs)

burnless diff old.yaml new.yaml
Comparing old.yaml → new.yaml
    service: payments-api (unchanged)
  ~ SLO: availability target: 99.90% → 99.95%
  + SLO: latency_p99 added (target: 99.5%)
Changes detected. Review before running burnless apply.

Tests

• TestDiffPreview_ValidFile
• TestDiffTwoFiles_DetectsChanges
• TestDiffTwoFiles_NoChanges