Remove Existing Stack Feature (Deprecated) and Introduce VM Feature

[yulmwu]

Previously, VM instances were provided through the in-house container-provisioner-k8s based on Kubernetes orchestration. However, this approach introduced unnecessary overhead and resulted in overengineering.

To address this, a custom orchestrator called sandboxd-o was developed, allowing isolated sandbox environments to be provided under the concept of VMs rather than Stacks.

This PR removes the previous Stack implementation and fully replaces it with the new VM-based model.

• Refs: Deprecated Stack Service and Added sandboxd-o Orchestrator Client for VM Feature wargame#13

[codecov]

Codecov Report

❌ Patch coverage is 75.20392% with 152 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/service/vm_service.go	59.68%	61 Missing and 41 partials ⚠️
internal/http/handlers/handler.go	73.61%	9 Missing and 10 partials ⚠️
internal/repo/vm_repo.go	86.56%	9 Missing and 9 partials ⚠️
internal/http/router.go	0.00%	9 Missing ⚠️
internal/vm/manifest.go	77.77%	2 Missing and 2 partials ⚠️

📢 Thoughts on this report? Let us know!

[Copilot]

Copilot encountered an error and was unable to review this pull request. You can try again by re-requesting a review.

[yulmwu]

Since this has already been validated in the existing Wargame environment, code review will be omitted.