[pi-extension] Observability & User Controls

[codenamekt]

Overview

Add user-visible controls and feedback for the hexus pi-extension.

Issues to Address

• No way to disable reflection mid-session — add hexus reflection-off command and HEXUS_REFLECTION_ENABLED runtime toggle
• No reflection history — store reflection metadata in hexus itself (what was extracted, when, turn count)
• No recall hit rate — track how often recalled memories are actually referenced in responses, surface in status
• No memory storage stats — show "N turns stored, M memories total" in status bar

Acceptance Criteria

• hexus reflection-off / hexus reflection-on commands work
• Reflection history stored in hexus with source=reflection-history target
• Status bar shows memory count and turn count
• Debug command hexus stats shows recall hit rate and storage stats