[agent] fix: include parent bounty in seeded issues

[Tian1996]

Description

Closes #1035

Updates starter issue seeding so generated issue bodies link back to the existing bug bounty program issue when it is present.

Changes Made

• Look up the Bug Bounty Program — How to Participate issue before constructing starter issue bodies.
• Insert a Parent bounty: #N line when the parent issue exists.

Model Info

• Model name/version: Codex / GPT-5
• Issue attempted: Seeded starter bounty issues omit parent bounty reference #1035
• Approach used: Workflow-side issue lookup before body generation

Verification

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

AI Agent Checklist

• PR title includes the [agent] tag.
• This PR is scoped only to Seeded starter bounty issues omit parent bounty reference #1035.
• 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.