fix: override set YAML format + cluster shared-values commands

[omattsson]

Summary

• Fix override set: CLI sends JSON map but backend expects YAML string #45: override set was sending values as a JSON map ({"values": {"key": "val"}}) but the backend expects a YAML string ({"values": "key: val"}). Changed SetValueOverrideRequest.Values to string and added yaml.Marshal serialization before sending.
• Closes Reduce per-instance override burden for local dev clusters #47: Added cluster shared-values list/set/delete commands so cluster-level Helm values can be configured once instead of repeated per-instance per-chart overrides. Supports --file, --set, --name, and --priority flags.

Changes

• pkg/types/types.go: SetValueOverrideRequest.Values changed from map[string]interface{} to string; added SharedValues and SetSharedValuesRequest types
• cmd/override.go: Added yaml.Marshal(values) before sending request
• pkg/client/client.go: Added ListSharedValues, SetSharedValues, DeleteSharedValues methods
• cmd/cluster.go: Added cluster shared-values list/set/delete commands
• Updated all tests (unit, client, integration, live) for the new string type
• Added 16 command tests and 8 client tests for shared-values
• README updated with new commands and corrected override quota flags

Test plan

• go test ./... -count=1 — all packages pass
• go vet ./... — clean
• Override set tests validate YAML string in request body
• Shared-values tests cover table/JSON/YAML/quiet output, file/set input, path traversal, confirmation flow, error handling
• Manual test against running backend with stackctl override set and stackctl cluster shared-values set

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