feat: burnless init interactive wizard (#12)

[Dinushi94]

What this PR does

Extends burnless init from a basic 2-question prompt into a full 6-step interactive wizard using the survey library.

Changes

• Added github.com/AlecAivazis/survey/v2 dependency
• Rewrote internal/cli/init.go to prompt for:
  1. Service name (required)
  2. Team (required)
  3. SLO availability target % (default 99.9, validated 0–100)
  4. SLO window — arrow-key select: 7d / 30d / 90d (default 30d)
  5. Prometheus metric name (default http_requests_total)
  6. Slack channel for alerts (default #sre-incidents)
  7. On-call provider — arrow-key select: pagerduty / opsgenie / none (default pagerduty)

Testing

• Smoke tested: all 6 prompts fire, sre.yaml written correctly
• burnless validate passes on generated file
• make lint clean

Closes #12