Add UserPromptSubmit sibling to the existing SessionStart activation
hook. SessionStart anchors the active output-style body once per
session; under sustained task pressure on long sessions even that
anchor fades because nothing re-injects it between turns.

The new hook fires on every user message and emits a short (~25 token)
reminder, keeping the style name + core rules at the top of attention
every turn. Pattern adapted from caveman.

Claude Code only - output styles are a Claude Code feature; Codex
hosts are unaffected.

Re-stamp with metalmind stamp to wire ~/.claude/settings.json.