feat: dedup guard — skip tasks targeting same GitHub issue as running task

[stackbilt-admin]

Context

When an upstream task queue (e.g. an issue-watcher) has a race condition, the taskrunner can end up executing two tasks for the same GitHub issue concurrently or sequentially. This results in duplicate PRs and merge conflicts.

Found in production: Stackbilt-dev/aegis#162

Proposed Enhancement

Before starting a task, the taskrunner should check the queue for other tasks with the same github_issue_repo + github_issue_number that are already running or completed. If found, skip the duplicate and mark it as cancelled with a reason.

This is a defense-in-depth measure — the upstream queue should prevent duplicates, but the runner should also guard against them.

Implementation Sketch

In execute_task(), after fetching the task but before starting Claude Code:

# Check for duplicate issue tasks
if [[ -n "$github_issue_number" && -n "$github_issue_repo" ]]; then
  existing=$(curl -s "${API_BASE}/api/cc-tasks?github_issue_repo=${github_issue_repo}&github_issue_number=${github_issue_number}&status=completed,running" | jq '.tasks | length')
  if [[ "$existing" -gt 0 ]]; then
    log "Skipping duplicate: another task for ${github_issue_repo}#${github_issue_number} already exists"
    # Cancel this task
    curl -s -X POST "${API_BASE}/api/cc-tasks/${task_id}/cancel" ...
    return 0
  fi
fi

This requires the task queue API to support filtering by github_issue_repo and github_issue_number, which the AEGIS implementation already does.