feat: D24 fact intervals (--as-of) and bin/stack assistant helpers. (#37)

Store valid_from/valid_to on leafs and filter search by calendar day without
overloading D16 source staleness; stack start/start-assistant wires brain + PicoClaw.

bin/stack/start-assistant

@@ -0,0 +1,15 @@
+#!/usr/bin/env bash
+# bin/stack/start-assistant - start + CPU reasoner + PicoClaw, then attach agent.
+#
+#   bin/stack/start-assistant
+#   bin/stack/start-assistant --no-attach
+#   bin/stack/start-assistant -- -m "search the 2dph brain for LadybugDB"
+#
+# Pulls qwen3.5:9b if missing. Gateway :18790. Agent uses MCP search → get → audit.
+# --no-attach leaves the gateway up without exec.
+set -euo pipefail
+
+STACK_DIR="$(CDPATH= cd -- "$(dirname "$0")" && pwd)"
+# shellcheck source=lib.sh
+source "$STACK_DIR/lib.sh"
+stack_start_assistant "$@"