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

@@ -0,0 +1,21 @@
+#!/usr/bin/env bash
+# bin/stack/start - bring up brain HTTP/MCP and wait until search/get/audit respond.
+#
+#   bin/stack/start
+#   bin/stack/status
+#
+# Reuses a healthy process on :8630 (host serve or compose). Does not start
+# PicoClaw. Does not rebuild Ladybug.
+set -euo pipefail
+
+STACK_DIR="$(CDPATH= cd -- "$(dirname "$0")" && pwd)"
+# shellcheck source=lib.sh
+source "$STACK_DIR/lib.sh"
+case "${1:-}" in
+-h | --help)
+	stack_usage "$0"
+	exit 0
+	;;
+esac
+stack_start "$@"
+stack_status