Surface agent code in OverallBestEntry for AgentDetailPage

[PunchTheDev]

Summary

GET /leaderboard/overall and GET /rounds/{id}/leaderboard now include agent_path + commit_hash on every per-spec best record. Closes the flywheel gap on the dashboard's AgentDetailPage — per-problem rows can now deep-link to the exact commit of code that achieved each score.

Motivation

Operator directive (cad-optimizations-todos): "you show the agent that achieved SOTA for each problem and their performance — but NOT their agentic code. The whole point is that the winning agent code gets open-sourced so the next person can fork it and beat it by a decaying margin. Surface the code."

HeroStats (spec detail), SotaHero (homepage), and Leaderboard (per-row) already link to agent code on GitHub. AgentDetailPage was the missing surface — OverallBestEntry carried score + submitted_at but no path/commit, so the per-problem result rows had nothing to link to.

Changes

• app/models.py: add agent_path: str and commit_hash: str to OverallBestEntry.
• app/routes/leaderboard.py: SELECT s.agent_path, s.commit_hash and pass through.
• app/routes/rounds.py: same change in the round-leaderboard path.
• CHANGELOG.md: 0.15.11 → 0.15.12.

24 existing tests in test_overall_leaderboard.py + test_rounds.py continue to pass — the new fields are populated from columns the fixtures already provide.