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

CONTEXT.md

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+# STS2 Bot
+
+Terms for observing Slay the Spire 2, choosing actions, and evaluating play.
+
+## Language
+
+**Run**:
+One game attempt, from its start to victory, death, or abandonment.
+A seed describes generated content; it does not uniquely identify an attempt.
+
+**Session**:
+One period of bot operation. A run can span sessions, and a session can span runs.
+
+**Observation**:
+The game information visible at one moment. It is not the complete game state.
+
+**Fact**:
+A directly observed value or a result established from known game rules and sufficient inputs.
+_Avoid_: Using this term for an unverified prediction.
+
+**Estimate**:
+A prediction that depends on assumptions or incomplete game information.
+_Avoid_: Guaranteed result, proven lethal
+
+**Decision**:
+A proposed next action. A decision does not establish that the game received or applied the action.
+
+**Action result**:
+The game's response to an attempted action. Acceptance alone does not establish that the expected change occurred.
+
+**Run deck**:
+The persistent collection of cards owned during a run, distinct from temporary combat cards and combat piles.
+_Avoid_: Treating all visible combat cards as the persistent deck.
+
+**Run outcome**:
+The result and progress of one game attempt. It does not establish whether each earlier decision was correct.
+
+**Decision label**:
+An assessment of one decision against a stated criterion, such as legality or an expert preference.
+_Avoid_: Treating the run outcome as an optimal-action label.