story-kit: create 178_story_fix_chat_textarea_input_lag

.story_kit/work/1_upcoming/178_story_fix_chat_textarea_input_lag.md

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+---
+name: "Fix chat textarea input lag"
+---
+
+# Story 178: Fix chat textarea input lag
+
+## User Story
+
+As a user, I want typing in the chat input to feel responsive, so that I can compose messages without frustrating delays.
+
+## Acceptance Criteria
+
+- [ ] Extract ChatInput as a separate component that owns its own input state, only calling parent on submit
+- [ ] Wrap calculateContextUsage in useMemo so it doesn't recompute on every keystroke
+- [ ] Extract MessageItem as a React.memo component so individual messages skip re-render on unrelated state changes
+- [ ] Typing in the textarea does not trigger re-render of the message list
+
+## Out of Scope
+
+- TBD