chore(phase3): fix mode wording drift in copilot-instructions.md

Align 'closure and normalization mode' → 'closure and hardening mode'
to match AGENTS.md and CLAUDE.md.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: LessUp

.github/copilot-instructions.md

@@ -1,6 +1,6 @@
 # Copilot instructions for this repository
 
-This repository is in **closure and normalization mode**.
+This repository is in **closure and hardening mode**.
 
 ## Priorities