Replaces cloudformation based stack discovery with tag based discovery

[sjysngh]

Note

Replaces CloudFormation outputs-based discovery with tag-based resource discovery via RGTA (with AppRunner fallback), updates doctor to use it, and documents the behavior.

• Core:
  • Resource discovery: Use AWS Resource Groups Tagging API with primary runs-on-stack-name tag and fallback via AppRunner service tag key.
  • Classification: Identify apprunner-service, config-bucket, and ec2-log-group via runs-on-resource tags or ARN-pattern fallbacks.
• Stack Doctor:
  • Remove CloudFormation dependency and stack status check; derive service URL and resources from discovered config; fetch logs and checks accordingly.
• Docs:
  • Add “Resource Discovery” section; update flag text from “CloudFormation stack name” to “Stack name”; refresh example outputs.
• Dependencies:
  • Bump aws-sdk-go-v2; add service/resourcegroupstaggingapi; update related internal packages (smithy-go, endpoints/configsources).

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[crohr]

@cursor review