feat: add architecture field to Package

[mgoldsborough]

Problem

When publishing MCPB packages that support multiple architectures, authors must create separate Package entries for each OS/architecture combination. Currently, clients have no standardized way to determine which package is appropriate for their platform without parsing the identifier URL.

For example, a server with Linux AMD64 and ARM64 builds requires two packages:

{
  "packages": [
    {
      "registryType": "mcpb",
      "identifier": "https://github.com/NimbleBrainInc/mcp-echo/releases/download/v0.1.0/mcp-echo-v0.1.0-linux-amd64.mcpb",
      "version": "0.1.0",
      "fileSha256": "26ef091c...",
      "transport": { "type": "streamable-http", "url": "http://localhost:8000/mcp" },
      "environmentVariables": [...]
    },
    {
      "registryType": "mcpb",
      "identifier": "https://github.com/NimbleBrainInc/mcp-echo/releases/download/v0.1.0/mcp-echo-v0.1.0-linux-arm64.mcpb",
      "version": "0.1.0",
      "fileSha256": "c5941f02...",
      "transport": { "type": "streamable-http", "url": "http://localhost:8000/mcp" },
      "environmentVariables": [...]
    }
  ]
}

Clients must currently either:

1. Parse the URL to guess the architecture (this is what we're doing now, but it's fragile with no standard format)
2. Iterate through all packages and try each one (inefficient)

Solution

Add an optional architecture field to the Package type that explicitly declares the target OS and architecture using the {os}-{arch} format (e.g., linux-amd64, darwin-arm64).

This follows the convention used by Go (GOOS-GOARCH), Docker, and other package ecosystems.

Changes

File	Change
pkg/model/constants.go	Add architecture constants (linux-amd64, linux-arm64, darwin-amd64, darwin-arm64, windows-amd64, windows-arm64)
pkg/model/types.go	Add optional Architecture field to Package struct
docs/reference/api/openapi.yaml	Add architecture property to Package schema
docs/reference/server-json/server.schema.json	Auto-regenerated

Example Usage

{
  "packages": [
    {
      "registryType": "mcpb",
      "architecture": "linux-amd64",
      "identifier": "https://github.com/.../mcp-echo-v0.1.0-linux-amd64.mcpb",
      "version": "0.1.0",
      "fileSha256": "26ef091c...",
      "transport": { "type": "streamable-http", "url": "http://localhost:8000/mcp" }
    },
    {
      "registryType": "mcpb",
      "architecture": "linux-arm64",
      "identifier": "https://github.com/.../mcp-echo-v0.1.0-linux-arm64.mcpb",
      "version": "0.1.0",
      "fileSha256": "c5941f02...",
      "transport": { "type": "streamable-http", "url": "http://localhost:8000/mcp" }
    }
  ]
}

Clients can now filter packages directly:

for _, pkg := range server.Packages {
    if pkg.Architecture == runtime.GOOS + "-" + runtime.GOARCH {
        return pkg // Found matching package
    }
}

Test plan

• Verify make check passes
• Review generated schema includes new field with examples