`evalcore init`: scaffold a new eval suite in one command

[abhishekmanyam]

Summary

Add evalcore init to scaffold a runnable eval suite in the current directory, so a newcomer's first successful run is one command away instead of hand-writing YAML.

Proposed behavior

evalcore init writes:

• evals.yaml — a minimal suite with a shell target (works with zero API keys) and a contains scorer.
• cases.jsonl — two or three example cases.
• A tiny bot.sh stub the shell target calls.

Then prints the next command (evalcore run evals.yaml). Optionally evalcore init --openai scaffolds an openai-compatible target variant instead.

Constraints

• Refuse to overwrite existing files unless --force.
• Output must be deterministic (no timestamps in generated files).
• The generated suite must pass evalcore run immediately, offline.

Good first issue

Mostly wiring in crates/evalcore (the bin) plus a CLI test with assert_cmd asserting the files land and the scaffolded suite runs green.