Fix JSON field mapping and response types

[omattsson]

Summary

• Fix JSON struct tags to match backend API field names (is_published, owner_id, last_deployed_at, repository_url, pageSize)
• Use ChartName instead of Name for chart display (backend has no name field)
• Change deploy/stop/clean return type to DeployResponse (matches backend 202 response)
• Change template instantiate to return StackDefinition (matches what the API creates)
• Rename CHARTS column to DEFINITIONS in template list, update help text

Test plan

• go test ./... -- all pass (unit, integration, e2e)
• stackctl template list -- PUBLISHED and DEFINITIONS columns populated
• stackctl template get -o json -- all fields present
• stackctl template instantiate --name test -- shows definition details
• stackctl stack deploy -- log ID shown correctly

Companion backend PR: omattsson/k8s-stack-manager#177

Fixes #38

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