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_precompact_hook.sh

@@ -68,10 +68,10 @@ if [ -n "$MEMPAL_DIR" ] && [ -d "$MEMPAL_DIR" ]; then
     python3 -m mempalace mine "$MEMPAL_DIR" >> "$STATE_DIR/hook.log" 2>&1
 fi
 
-# Always block — compaction = save everything
+# Notify — compaction is about to happen but filing is handled in background
 cat << 'HOOKJSON'
 {
-  "decision": "block",
-  "reason": "COMPACTION IMMINENT. Save ALL topics, decisions, quotes, code, and important context from this session to your memory system. Be thorough — after compaction, detailed context will be lost. Organize into appropriate categories. Use verbatim quotes where possible. Save everything, then allow compaction to proceed."
+  "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