feat: add Cursor CLI agent harness and experiments

[vivek100]

Summary

• Add a Cursor CLI harness (packages/core/src/agents/cursor/) with pinned install, headless stream-json runner, transcript parser, and cursorAgent() factory wired into the registry.
• Add four experiments mirroring Claude Code: cursor-composer-2.5, cursor-composer-2.5-no-skills, cursor-grok-4.5-high, cursor-grok-4.5-high-no-skills (Supabase MCP on; skills vs none).
• Document CURSOR_API_KEY in .env.example / README and label Cursor in the results UI.

Fixes #167

How to run

# .env needs CURSOR_API_KEY; OPENAI_API_KEY for rubric/judge checks
pnpm eval -- --experiment cursor-composer-2.5 --eval resolve-dataapi-001-empty-results

Model ids (from live cursor-agent --list-models): composer-2.5, cursor-grok-4.5-high.

Verification

• Unit: pnpm exec vitest run src/agents/cursor/ (core package)
• Typecheck / format: pnpm typecheck, pnpm format:check
• Discovery: pnpm eval:dry for all four Cursor experiments
• Tools-mode smoke: cursor-composer-2.5 × resolve-dataapi-001-empty-results — agent completed with Supabase MCP (list_tables, execute_sql, apply_migration, …) and skills loaded. Scorer judge step needs OPENAI_API_KEY (eval uses judge() in addition to deterministic checks).

Test plan

• pnpm --filter @supabase-evals/core test (includes Cursor runner/parser tests)
• pnpm typecheck
• Dry-run one Cursor experiment
• Optional: add run-evals label if maintainers want full benchmark CI
• Optional: targeted eval-refresh for the new Cursor experiments

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