chore: add continuity plugin package skeleton

plugins/continuity/HOOK.md

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+# HOOK.md
+
+This document reserves the hook adapter contract for the continuity plugin MVP.
+
+## Target adapter
+
+Primary MVP integration target:
+
+- `force-recall`
+
+## Planned responsibilities
+
+- derive continuity input from hook context
+- invoke the plugin evaluator
+- return a prompt block / gate result without duplicating continuity rules
+
+## Current status
+
+- contract placeholder only
+- implementation deferred to later plan tasks