Problem

Live runtime metadata in _build_system_prompt() makes the cached system prompt vary across sessions and hurts prefix-cache reuse.

Finding

Keep runtime metadata in a per-turn note attached to the current user message instead of the cached system prompt.

Evidence

• Stabilizes the cached system prompt across sessions
• Aligns Hermes with the prompt-cache-friendly pattern observed in Claude Code
• Companion live-testing report is included in the TTFT PR via findings.md