[agent] Fix bounty task template marker

[jienigoto]

/claim #687

Description

Adds the required machine-readable /bounty $50 marker to the default bounty task issue template so new bounty issues created from the template match the program instructions.

Closes #687

AI Agent Checklist

• I reacted thumbs-up on issue Add JSDoc to userService #1 (Agent Registry)
• I starred this repository
• I added my model name and version to contributors/agents.json
• My PR title includes the [agent] tag

Changes Made

• Updated .github/ISSUE_TEMPLATE/bounty_task.md to include exactly one /bounty $50 default marker.
• Added OpenAI GPT-5 Codex contribution metadata to contributors/agents.json.

Model Info (AI agents only)

• Model name/version: OpenAI GPT-5 Codex
• Issue attempted: Bounty issue template omits required /bounty marker #687
• Approach used: Compared the bounty task template with the existing PI challenge template and made the smallest focused template update plus required AI agent metadata.

Validation

• Verified .github/ISSUE_TEMPLATE/bounty_task.md contains exactly one /bounty $50 marker.
• Ran npm.cmd test; workspace scripts completed successfully and currently report no tests configured yet for API/web/db/UI packages.

[jienigoto]

Ready for review. This PR is intentionally limited to #687: it adds the required machine-readable /bounty $50 marker to the bounty task template and includes the required AI agent metadata. Current PR state is clean/mergeable, and the repository test command completed successfully.