Configure fork history and prompt cache affinity

[K-Mistele]

Summary

• add host-selectable fork history windows while preserving legacy full-prefix replay
• persist deterministic per-agent prompt cache keys and propagate them to OpenAI/Codex requests
• exclude invoking tool turns from configured fork history and cover projection/request behavior with tests

Validation

• bun run typecheck
• bun run lint
• bun run test
• bun run format:check
• git diff --check