Migrate AskForCluster from survey to huh

[blsmth]

Summary

• Migrate AskForCluster shared helper from survey to huh
• Extract ClusterSelectForm as a pure form builder function
• Change AskForCluster signature from response interface{} to *string for type safety
• Skip prompt when --cluster flag is already provided (fixes apppack app create --cluster flag still prompts for cluster selection #115)
• Update all 3 callers (database, redis, app_pipeline)
• Remove clusterSelectTransform — replaced by typed huh.NewOption(displayName, stackName)

Closes #120
Fixes #115

Changes

File	Purpose
stacks/stacks.go	Replace survey with huh; extract ClusterSelectForm; skip prompt when flag set
stacks/stacks_test.go	teatest tests for cluster selection
stacks/database.go	Update caller to pass &a.Parameters.ClusterStackName
stacks/redis.go	Update caller to pass &a.Parameters.ClusterStackName
stacks/app_pipeline.go	Update caller to pass &a.Parameters.ClusterStackName