Add hello workflow as a minimal hello world example

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

arc/workflows/hello/workflow.dot

@@ -0,0 +1,11 @@
+digraph Hello {
+    graph [goal="Say hello and demonstrate a basic arc workflow"]
+    rankdir=LR
+
+    start [shape=Mdiamond, label="Start"]
+    exit  [shape=Msquare, label="Exit"]
+
+    greet [label="Greet", prompt="Say hello! Introduce yourself and explain that this is a test of the arc workflow engine."]
+
+    start -> greet -> exit
+}

arc/workflows/hello/workflow.toml

@@ -0,0 +1,5 @@
+version = 1
+graph = "workflow.dot"
+
+[sandbox]
+provider = "local"