Identity prompt should not overwrite existing GitHub secrets #121

Problem

The identity prompt that sets up the GitHub ↔ Azure connection may overwrite existing GitHub repository secrets. For example, if a user already has a secret named AZURE_SUBSCRIPTION_ID for other workflows, the identity prompt will silently overwrite it, potentially breaking their existing CI/CD pipelines.

Proposed Solution

Update the identity prompt to check for existing secrets and interactively prompt the user before writing:

1. Check if the secret already exists — before setting each secret (e.g., AZURE_SUBSCRIPTION_ID, AZURE_CLIENT_ID, AZURE_TENANT_ID), query the GitHub API to see if a secret with that name is already present
2. If a conflict exists, prompt the user — show the user which secrets already exist and ask whether they want to:
   • Overwrite the existing secret (e.g., if it's from a previous APIOps setup)
   • Use a different name (e.g., APIOPS_AZURE_SUBSCRIPTION_ID) — let the user confirm or customize the alternative name
   • Reuse the existing secret — if the existing value is already correct, skip setting it
3. Update pipeline references — ensure the scaffolded pipeline YAML files reference the correct secret names based on the user's choices
4. Inform the user — summarize what was set and what was skipped/renamed

Acceptance Criteria

• Identity prompt checks for existing GitHub secrets before creating new ones
• If a conflict is detected, the user is prompted with options (overwrite, rename, or reuse)
• User can choose or customize alternative secret names
• Scaffolded pipeline files reference the correct secret names based on user choices
• Existing secrets are never silently overwritten