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/status

@@ -0,0 +1,17 @@
+#!/usr/bin/env bash
+# bin/stack/status - YAML health for brain MCP, reasoner model, PicoClaw gateway.
+#
+#   bin/stack/status
+#   bin/stack/status | yq '.picoclaw'
+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_status