Add ensureLabelsExist() utility function to github.ts

[helixclaw]

Summary

Add a new ensureLabelsExist() function in src/utils/github.ts that checks whether labels exist on a GitHub repo and creates any missing ones.

Context

Child issue of #12. This is the core utility function.

Implementation Details

Function signature

export async function ensureLabelsExist(
  labels: string[],
  repo?: string
): Promise<string[]>  // returns labels that exist or were successfully created

Behavior

1. For each label, call gh api repos/{owner}/{repo}/labels/{label_name} to check existence (URL-encode the label name)
2. If a label doesn't exist (404), create it with gh api -X POST repos/{owner}/{repo}/labels -f name={label} -f color=ededed
3. If label creation fails (e.g., 403 permission denied), log a warning and exclude it from the returned list rather than failing
4. Return the list of labels that exist or were successfully created

Error Handling

• Label check fails (network error): warn and skip label
• Label creation fails (permission denied): warn and skip label
• Label already exists: include in return list

Acceptance Criteria

• Function checks for label existence via GitHub API
• Missing labels are created with a default color
• Permission errors handled gracefully (warn + skip)
• Works with explicit repo parameter and without (current repo)
• Unit tests with mocked executeGhCommand covering all paths