Root-level node creation fails and add timestamp support

[sreuter]

I noticed that creating nodes at the root level doesn't work - turns out the upstream workflowy package sends "Root" instead of "None" as the parentid. The create_node tool was also missing its inputSchema which broke MCP.

While debugging this I also found that operations on nested nodes fail since the code only searches top-level items.

I also added support for exposing timestamps (createdAt, lastModifiedAt, completedAt) from the workflowy API.

Upstream issues:

• Root fix: Root-level node creation fails - wrong parentid sent karelklima/workflowy#11
• Timestamps: Add created timestamp and expose timestamps in toJson karelklima/workflowy#13

Fix: #6