fix: replace invalid 'decision: allow' with {} in hooks

Closes #872. The top-level decision field only recognizes "block".
To not block, return empty JSON {}. "allow" was silently ignored
by Claude Code, causing unpredictable behavior.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hooks/mempal_precompact_hook.sh

@@ -68,10 +68,6 @@ if [ -n "$MEMPAL_DIR" ] && [ -d "$MEMPAL_DIR" ]; then
     python3 -m mempalace mine "$MEMPAL_DIR" >> "$STATE_DIR/hook.log" 2>&1
 fi
 
-# Notify — compaction is about to happen but filing is handled in background
-cat << 'HOOKJSON'
-{
-  "decision": "allow",
-  "reason": "MemPalace pre-compaction save. Your full conversation has been saved verbatim in the background — no action needed. Compaction can proceed safely."
-}
-HOOKJSON
+# Silent: return empty JSON to not block. "decision": "allow" is invalid —
+# only "block" or {} are recognized.
+echo '{}'