fix: add cycle detection to prevent infinite recursion in graphExecutor

[g3Bg2]

Fixes #79

What was broken

executeGraph() in graphExecutor.ts tracks visited nodes via a Set<string> but never checks it before recursing into a node. If a workflow graph contains a cycle (e.g., Node A → Node B → Node A), the executor would recurse infinitely until the 10,000-iteration guard fired — at which point the automation crashes with a generic error instead of detecting the cycle early.

What was changed

Added a single guard at the top of executeGraph() (before any other logic):

if (visited.has(nodeId)) return;

This causes the executor to skip any node it has already visited, short-circuiting cycles cleanly and immediately — without crashing or burning through the iteration budget.

How to test

1. Create a workflow with a cycle: e.g., two nodes connected in a loop.
2. Run the automation.
3. Before fix: automation hangs until hitting the 10k iteration guard, then crashes.
4. After fix: automation executes each node once and exits cleanly.