#823 Expression editor + variable picker for the visual workflow editor

Add n8n-style expression support to the visual editor's inspector fields. Highway interpolates {{ variable_path }} inline in any string value (safe_eval _VARIABLE_PATTERN); root = an upstream task's result_key (shell auto-unwraps stdout; sub-paths {{key.stdout}} work), plus {{workflow_run_id}} and {{ENV.NAME}}. Build: (1) expressions.ts — compute available refs for a node (ancestor tasks' result_keys via edge traversal + built-ins); (2) ExpressionField.svelte — text/textarea + a '{ }' picker button that inserts {{ref}} at the cursor, grouped Upstream/Other/Built-in with filter; (3) wire into ParamForm string widgets + InspectorPanel condition/switch_on/items fields. Note: picker inserts result_key refs, NOT task_id (task_id refs like the switch_demo ${...} don't resolve).