Implement --token and MCP_GITHUB_TOKEN login support (#705)

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## Motivation and Context
<!-- Why is this change needed? What problem does it solve? -->
The following PR implements the missing support for logging in via
`--token` or `MCP_GITHUB_TOKEN`

## How Has This Been Tested?
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tested? -->

## Breaking Changes
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## Types of changes
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the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update

## Checklist
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that apply. -->
- [ ] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [ ] My code follows the repository's style guidelines
- [ ] New and existing tests pass locally
- [ ] I have added appropriate error handling
- [ ] I have added or updated documentation as needed

## Additional context
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Fixes: #698

---------

Signed-off-by: Radoslav Dimitrov <radoslav@stacklok.com>

cmd/publisher/commands/init.go

@@ -29,7 +29,7 @@ func InitCommand() error {
 	description := detectDescription()
 	version := "1.0.0"
 	repoURL := detectRepoURL()
-	repoSource := "github"
+	repoSource := MethodGitHub
 	if repoURL != "" && !strings.Contains(repoURL, "github.com") {
 		if strings.Contains(repoURL, "gitlab.com") {
 			repoSource = "gitlab"