feat(template): version list/get/diff

[omattsson]

Summary

• GET /api/v1/templates/:id/versions → stackctl template versions list <id>
• GET /api/v1/templates/:id/versions/:versionId → stackctl template versions get <id> <version>
• GET /api/v1/templates/:id/versions/diff?from=&to=→ stackctl template versions diff <id> <v1> <v2>

Files to touch

• cli/pkg/types/types.go — add TemplateVersion, TemplateVersionDiff.
• cli/pkg/client/client.go — add three methods.
• cli/cmd/template.go — add nested versions subcommand (mirror cluster shared-values pattern in cli/cmd/cluster.go).

Tests (all three layers required)

• Unit: cli/cmd/template_test.go + cli/pkg/client/client_test.go — list/get/diff with all output modes; diff renders unified text in table mode and structured JSON in json/yaml.
• Integration: cli/test/integration/template_definition_integration_test.go — Cobra in-process list+get+diff round-trip.
• E2E: cli/test/e2e/cli_e2e_test.go — one happy-path scenario for template versions diff.

Acceptance

• diff output: unified text (table mode) and structured JSON (json/yaml).

Tracks #59