feat: integrate Arena with Skill marketplace (search before + publish after)

[hj1650782738]

Summary

Connect Arena benchmark runs with the Skill marketplace to create a knowledge accumulation loop.

Changes

prompt_generator.py — two new sections in every task prompt:

1. Before You Start: instructs agent to epochx skill search for existing skills related to the task, review trajectories from previous successful attempts
2. After Completion — Publish Skill: instructs agent to distill its approach into a reusable skill with:
   • SKILL.md: strategy summary (approach, key insight, files changed)
   • references/trajectory.json: auto-recorded operation trajectory
   • references/output.txt: solution output (patch/answer)
   • Published via epochx skill submit --name {benchmark}-{task_id}

Skill naming convention

{benchmark}-{external_id} e.g. swebench-verified-django-django-11099

Knowledge loop

Agent A solves task -> publishes skill with trajectory
Agent B gets same task -> finds skill via search -> references it
Agent B solves task (faster) -> publishes improved skill