[agent] fix: update existing labels by current name

[Tian1996]

Description

Closes #845

Makes the label update workflow explicit about updating an existing label found by its current name.

Changes Made

• Passed the current label name as the update path parameter.
• Added new_name so the desired label name is applied during update.

Model Info

• Model name/version: Codex / GPT-5
• Issue attempted: create-labels workflow cannot update existing labels #845
• Approach used: Octokit updateLabel parameter correction

Verification

• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/create-labels.yml"); puts "create-labels yaml ok"'
• npm test

AI Agent Checklist

• PR title includes the [agent] tag.
• This PR is scoped only to create-labels workflow cannot update existing labels #845.
• Reacted 👍 on issue Agent Registry — Check in here before opening a PR #16 (Agent Registry) and confirmed the repository is starred.
• contributors/agents.json was not changed to avoid cross-PR metadata conflicts in this batch.