Add GitHub Actions CI/CD, staging, and production workflows

[kiro-agent]

This pull request was generated by @kiro-agent 👻

Comment with /kiro fix to address specific feedback or /kiro all to address everything.
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Summary

Creates three GitHub Actions workflow files in .github/workflows/:

1. ci.yml - CI Pipeline

• Triggers on pushes to main and pull requests
• Runs: flutter pub get, flutter analyze, flutter test, compliance check, flutter build web
• Includes concurrency control to cancel redundant runs

2. promotion.yml - Staging Deployment

• Triggers on push to main and manual workflow_dispatch
• Two-job pipeline: build-and-test, then deploy to staging
• Uses GitHub staging environment
• Deploys via CDK (KiroMobile-Staging stack)
• Writes CDK exports to job summary as a formatted markdown table with emoji indicators for URLs, API keys, buckets, distributions, and ARNs
• Includes collapsible raw JSON output and artifact upload

3. production.yml - Production Deployment

• Manual workflow_dispatch only, requires explicit "yes" confirmation and a git ref
• Three-job pipeline: validate inputs, build-and-test, then deploy
• Uses GitHub production environment (designed for required reviewers/approval gates)
• Deploys via CDK (KiroMobile-Production stack)
• Writes CDK exports to job summary with deployment metadata (ref, actor, timestamp) plus the same formatted exports table
• Input sanitization via env variables to prevent expression injection

CDK Exports Summary Format

Both deployment workflows parse cdk-outputs.json and render:

• A markdown table with emoji-classified exports (🔗 URLs, 🔑 API keys, 📦 S3 buckets, 🌐 CloudFront, 🏷️ ARNs)
• A collapsible <details> block with the raw JSON
• Graceful handling when no CDK outputs file exists

Notes

• AWS credentials are configured via OIDC (aws-actions/configure-aws-credentials@v4) - the IAM role ARN and region must be set in GitHub environment secrets
• CDK deploy steps are placeholders until the CDK app/stack definitions are added to the repo
• All workflows validated with yamllint (0 errors, 0 warnings)