#821 Visual workflow editor rebuild: Svelte Flow + real tools + correct compiler + round-trip

The dashboard visual editor is useless: dsl-generator.convertNodeToTask only emits task+parallel and silently DROPS condition/switch/while/foreach/join/events; tool config is 8 hardcoded names + raw-JSON args (no shell command box); DSL import is a TODO stub (no round-trip). Rebuild on @xyflow/svelte with backend GET /api/v1/tools (curated schemas + generic fallback), correct compiler for every operator (labeled-handle branching), parser for full round-trip, schema-driven inspector. Plan approved. Waves A-F.