Eval: add negative and ambiguity test cases

[olaservo]

Problem

All 7 eval tasks are happy-path activation tests. There are no tests for:

1. Negative tests — prompts that should NOT trigger any skill (e.g., "What's the weather?")
2. Ambiguity tests — prompts that could match multiple skills
3. Error paths — misspelled skill names, invalid inputs

The EvalConfig interface only supports positive assertions (expectedSkillName, expectedOutput). There's no way to express "this should NOT activate."

Suggestion

• Add shouldNotActivate?: boolean to EvalConfig
• Add unexpectedSkillNames?: string[] for skills that should NOT fire
• Update analyzeSession() to handle negative assertions
• Create task configs like no-skill.json, ambiguous.json

Files

• evals/lib/eval-checker.ts (EvalConfig interface, lines 5-9)
• evals/tasks/ (new task configs needed)