Bug: Go runtime setup fails when go.mod doesn't exist

[Mossaka]

Problem

The compiler automatically adds a "Setup Go" step when it detects Go commands (e.g., go version in custom steps or Serena Go language service), but this step fails if the repository doesn't have a go.mod file.

Root Cause

In pkg/workflow/runtime_setup.go:

1. Go runtime definition (lines 77-84):

   {
    ID: "go",
    Name: "Go",
    ActionRepo: "actions/setup-go",
    ActionVersion: "v5",
    VersionField: "go-version",
    DefaultVersion: "", // Special handling: uses go.mod
    Commands: []string{"go"},
   }

2. Setup step generation (lines 575-583):

   if runtime.ID == "go" && (version == "" || req.GoModFile != "") {
    step = append(step, " with:")
    goModPath := "go.mod" // ⚠️ Assumes go.mod exists
    if req.GoModFile != "" {
    goModPath = req.GoModFile
    }
    step = append(step, fmt.Sprintf(" go-version-file: %s", goModPath))
    step = append(step, " cache: true")
    // ...
   }

Result: The generated workflow uses go-version-file: go.mod, which fails if the file doesn't exist:

Error: The specified go version file at: go.mod does not exist

Steps to Reproduce

1. Create a workflow that uses Claude with Serena Go language service or has a custom step with go version
2. Run it in a repository without a go.mod file
3. The workflow fails during the "Setup Go" step

Proposed Fix

Add a default Go version and use it as a fallback:

1. Add to pkg/constants/constants.go:

   // DefaultGoVersion is the default version of Go for runtime setup
   const DefaultGoVersion Version = "1.23"

2. Update pkg/workflow/runtime_setup.go:82:

   DefaultVersion: string(constants.DefaultGoVersion),

3. Update logic at runtime_setup.go:575 to only use go-version-file when explicitly specified:

   if runtime.ID == "go" && req.GoModFile != "" {
    // Only use go-version-file if explicitly specified
    step = append(step, " with:")
    step = append(step, fmt.Sprintf(" go-version-file: %s", req.GoModFile))
    step = append(step, " cache: true")
   } else if version != "" {
    // Use explicit version
    step = append(step, " with:")
    step = append(step, fmt.Sprintf(" go-version: '%s'", version))
   }

This way:

• Repos with go.mod can explicitly specify go-mod-file in frontmatter
• Repos without go.mod will use the default Go version
• Users can still override with explicit version in frontmatter

Related Code

• Detection: pkg/workflow/compiler_yaml.go:246 calls DetectRuntimeRequirements()
• Generation: pkg/workflow/compiler_yaml.go:263 calls GenerateRuntimeSetupSteps()
• Runtime setup: pkg/workflow/runtime_setup.go:169-224 (detection) and pkg/workflow/runtime_setup.go:443-453 (generation)