New nodes: Sub-workflow, Notes, Slack, and Code

Four new nodes are live in the workflow builder:

Run Workflow - trigger a sub-workflow from within any workflow, with a depth cap to prevent infinite loops.

Notes - add inline documentation to any workflow; the node renders its text and passes it as output, so it doubles as a lightweight data node.

Slack - send messages to a Slack channel directly from a workflow step, no Zapier or Make required.

Code - run custom logic in a sandboxed eval environment. Use it for transformations, conditionals, or anything the standard nodes don't cover.

All four are additive — no existing workflows are affected.