test: add unit test suite for core utility modules

[Oxygen56]

Summary

Adds initial test infrastructure for SkillOpt with unit tests covering:

• skillopt/utils/scoring.py — compute_score and skill_hash
• skillopt/utils/json_utils.py — extract_json and extract_json_array
• skillopt/types.py — Edit, Patch dataclass serialization round-trip

These utility modules are pure functions with deterministic behavior, making them ideal starting points for test coverage. All tests pass without requiring any external API keys or LLM services.

Test Plan

• pytest tests/ -v — all tests pass
• No regressions in existing functionality

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