Include system prompt prefix (first 200 chars) in session key derivation.
Claude Code sessions have unique system prompts containing cwd, session_id,
file paths, etc., making collisions between concurrent sessions from the
same user virtually impossible.
Session key now = SHA256(apiKey + model + systemPrompt[:200] + firstUserMsg)
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>