fix(strategy): treat opened pack cost as sunk, not refundable

[BGarber42]

Summary

• Teach all strategies that after buying a booster, the cost is already spent — skipping does not refund money, so prefer taking upside unless there is a skip-payoff joker (e.g. Red Card) or every option is actively harmful.
• Fix pack card modifier rendering to use card.modifier.* so Standard packs show enhancement/edition/seal to the LLM.
• Add unit tests covering sunk-cost guidance and modifier rendering across default/conservative/aggressive.

Test plan

• Run uv run pytest tests/unit/test_strategy.py::TestPackOpenSunkCostGuidance tests/unit/test_strategy.py::TestRenderPackPlayingCardModifiers -v
• Spot-check a shop run: buy a pack, confirm prompts discourage skip-for-savings reasoning