Add page-level Fork CTA to agent profile header

[PunchTheDev]

Summary

Adds a context-aware Fork CTA row to the bottom of the agent-profile header card on /rankings/:agentId. A would-be competitor landing on the page now sees a clear, page-level action ("challenge this agent's setup") and an accent-styled Fork repo → button — separate from (and complementing) the per-problem ↗ code chips already in the table below.

Motivation

BACKLOG row Fork CTAs — ○ ○ ○. Before this PR the only fork affordances on the agent profile lived inside the per-row table chips. A would-be miner scanning the page chrome got no obvious "what do I do next" handle — failed the "would-be miner" seat.

Changes

• src/App.tsx L2185–2204 (AgentDetailPage): new bordered-top row inside the header card with two parts:
  • Sentence — branches on entry.total_wins:
    • total_wins > 0: Challenge {contributor}'s setup — fork the repo, open any of their N #1 problems below, and beat the score to take the top spot.
    • total_wins === 0: Race {contributor} for the top spot — fork the repo, pick any of their M entered problems below, and submit a passing agent with a better score.
  • Button — accent-bordered Fork repo → linking to ${FORGE_REPO}/fork, target=_blank, with a 155-char tooltip explaining what fork actually does ("Opens the GitHub fork dialog for PunchTheDev/forge — your fork is where you edit an agent.py, run forge eval locally, and open a PR back to this benchmark.")
• BACKLOG.md: Fork CTAs row flips ○ ○ ○ → ● ● ● with full notes on the implementation.

Verification

Puppeteer at 1440×900 on /rankings/PunchTheDev:

• ctaPresent=true
• ctaHref="https://github.com/PunchTheDev/forge/fork"
• target="_blank"
• ctaTooltipLen=155
• sentenceText="Challenge PunchTheDev's setup — fork the repo, open any of their 3 #1 problems below, and beat the score to take the top spot." (total_wins=3 path)
• accent bg rgba(99, 102, 241, 0.1) (forge-accent/10)
• borderTop=1px divider present
• 0 new console errors