Integrate ensureLabelsExist() into createIssue() flow

[helixclaw]

Summary

Wire up ensureLabelsExist() into the createIssue() flow so labels are auto-verified/created before gh issue create.

Context

Child issue of #12. Depends on #13 (ensureLabelsExist implementation).

Implementation Details

Changes to createIssue() in src/utils/github.ts

Before the gh issue create command is assembled, add:

let validLabels = labels
if (labels && labels.length > 0) {
  validLabels = await ensureLabelsExist(labels, repo)
}
if (validLabels && validLabels.length > 0) {
  args.push('--label', validLabels.join(','))
}

This replaces the current block at ~line 271:

if (labels && labels.length > 0) {
  args.push('--label', labels.join(','))
}

Acceptance Criteria

• createIssue() calls ensureLabelsExist() when labels are provided
• Only validated labels are passed to gh issue create
• Issue creation succeeds even when some labels can't be created (graceful degradation)
• createIssueInRepo() wrapper also benefits (delegates to createIssue())
• Integration test covering the end-to-end flow