fix(kugetsu): init creates base session in ~/.kugetsu-worktrees, adds context to forked sessions, and clears logs

1. Init: cd to ~/.kugetsu-worktrees before creating base session
   This keeps all worktrees inside a predictable directory structure
   and avoids external_directory permission issues

2. Init: Clear old logs but keep repos.json, config, and env files

3. Fork context: Add kugetsu_get_fork_context() that provides:
   - Important working rules (stop on error, don't pivot)
   - Repository configuration from repos.json
   - Environment file location info

4. Fork message: Prepend context to user message when forking session

skills/kugetsu/scripts/kugetsu

@@ -1123,6 +1123,11 @@ EOF
         echo "Created pm-agent env file: $ENV_DIR/pm-agent.env"
     fi
 
+    if [ -d "$LOGS_DIR" ]; then
+        echo "Cleaning up old logs..."
+        rm -rf "$LOGS_DIR"/*.log 2>/dev/null || true
+    fi
+
     local existing_base=$(get_base_session_id)
     local existing_pm=$(get_pm_agent_session_id)