Add friendly README with quick start and overview

[ScuttleBot]

Adds a welcoming README.md that:

• Opens with a clear value prop and link to the leaderboard
• Shows the hero image
• Explains why PinchBench exists (tests real-world agent tasks, not synthetic benchmarks)
• Provides quick start commands with run.sh wrapper
• Summarizes the 23 tasks by category in a scannable table
• Documents all CLI flags
• Explains how to submit results to the leaderboard
• Links to task contribution guide
• Keeps the crab/lobster theming 🦀🦞

The SKILL.md already has detailed docs for OpenClaw users — this README is aimed at people discovering the repo on GitHub.