Skip npm install when package.json is absent

Summary

• Add a shared dependency install helper that skips npm install when fixtures do not include package.json.
• Use the helper across Claude Code, Codex, OpenCode, Cursor, and Gemini adapters.
• Add unit coverage for skip/install/retry behavior and a patch changeset.

Test plan

• npm run test -w packages/agent-eval -- src/lib/agents/shared.test.ts
• npm run build -w packages/agent-eval
• npm run lint -w packages/agent-eval