fix(template): preserve separated manifests and assets

[mlhiter]

Summary

Fix template deployments and catalog/instance asset responses for repositories that separate template metadata from Kubernetes manifests. The provider now loads nested manifests/**/*.yaml files, excludes them from catalog indexing, and proxies repository-hosted icons through a browser-compatible asset endpoint. The template repository layout is documented and focused unit coverage is included.

Verification

• git diff --check origin/release-v5.1...HEAD passed.
• The template build could not run because the existing workspace dependency @sealos/gtm is unavailable, and Vitest is not declared or installed in the workspace.

Sequence Diagram

sequenceDiagram
    participant Browser
    participant TemplateAPI
    participant Repo as Template Repository
    participant AssetAPI

    Browser->>TemplateAPI: Request template or instance data
    TemplateAPI->>Repo: Read index metadata and nested manifests
    Repo-->>TemplateAPI: Return deployment source and asset references
    TemplateAPI->>AssetAPI: Rewrite repository icon URL
    AssetAPI-->>Browser: Serve icon with browser-compatible response
    TemplateAPI-->>Browser: Return separated deployment source and metadata

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