fix: stop hooks from making agents write in chat — save tokens

The save hook and precompact hook were telling the agent to write
diary entries, add drawers, and add KG triples IN THE CHAT WINDOW.
Every line written stays in conversation history and retransmits on
every subsequent turn — ~$1/session in wasted tokens.

Fix: hooks now say "saved in background, no action needed" and use
decision: allow instead of block. The agent continues working without
interruption. All filing happens via the background pipeline.

Also updated hooks README with:
- Known limitation: hooks require session restart after install
- Updated cost section: zero tokens, background-only

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

hooks/mempal_save_hook.sh

@@ -140,12 +140,15 @@ if [ "$SINCE_LAST" -ge "$SAVE_INTERVAL" ] && [ "$EXCHANGE_COUNT" -gt 0 ]; then
         python3 -m mempalace mine "$MEMPAL_DIR" >> "$STATE_DIR/hook.log" 2>&1 &
     fi
 
-    # Block the AI and tell it to save
-    # The "reason" becomes a system message the AI sees and acts on
+    # Notify the AI that a checkpoint happened — but do NOT ask it to write
+    # anything in chat. All filing happens in the background via the pipeline.
+    # The old version asked the agent to write diary entries, add drawers, and
+    # add KG triples in the chat window — that cost ~$1/session in retransmitted
+    # tokens and cluttered the conversation.
     cat << 'HOOKJSON'
 {
-  "decision": "block",
-  "reason": "AUTO-SAVE checkpoint. Save key topics, decisions, quotes, and code from this session to your memory system. Organize into appropriate categories. Use verbatim quotes where possible. Continue conversation after saving."
+  "decision": "allow",
+  "reason": "MemPalace auto-save checkpoint. Your conversation is being saved verbatim in the background — no action needed from you. Continue working."
 }
 HOOKJSON
 else