docs(dev): add lean agent guidance and prototype hardening research

.pi/prompts/sts2-check.md

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+---
+description: Run offline STS2 development checks and summarize failures
+---
+Read AGENTS.md and inspect `jj status` before checking the current workspace.
+Run these commands from the repository root:
+
+```sh
+python3 test_facts.py && python3 test_brain.py && python3 test_run.py
+python3 migrate.py --check-only
+bash -n eval_batch.sh ab_card_skip.sh
+```
+
+Run the dataset and shell checks even if a Python test fails.
+Do not run the game loop, call Jev, rebuild the dataset, or modify source files.
+Report commands, exit codes, test totals, and any environment-dependent skips.
+Finish with `jj status` and identify any unexpected workspace changes.

.pi/prompts/sts2-review.md

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+---
+description: Review STS2 changes for regressions and data integrity
+argument-hint: "[focus]"
+---
+Read AGENTS.md, then inspect `jj status` and `jj diff`.
+Review the current change. If the change is empty, review the requested area of the current code.
+Additional focus: ${ARGUMENTS:-game-loop safety and dataset integrity}
+
+Check fresh observations after actions, card-index validity, deterministic arithmetic,
+model-free fallbacks, confidence gates, transport failures, and dry-run side effects.
+Check session/step attribution, state/action linkage, and separation of run outcomes from correctness labels.
+Use offline reproductions where useful. Do not call live services or modify files.
+
+Report actionable findings first, ordered by severity, with file/line references and missing regression tests.
+Separate confirmed defects from proposed improvements. State review limits and checks performed.