Validate complete decision-tree node shapes before MCP persistence

[jamditis]

Problem

The LLM Advisor Data MCP server validates only the outer decision-node shape before persisting a new node. Nested options are accepted without enforcing the browser application's actual contract.

For example, a schema-valid option containing "tools": [] can be written successfully, but the browser later dereferences option.tools[0].name while populating the workflow selector. That aborts advisor initialization for every visitor. Other malformed nested tools values can also reach rendering code.

This was identified in the capped round-nine review of branch agent/tools-impact-burndown and is intentionally deferred from that issue-burndown PR.

Acceptance criteria

• Define and enforce the complete nested decision-tree option/tool shape before persistence.
• Reject empty or malformed tools arrays and unsupported fields.
• Validate both existing file contents and add_decision_node inputs against the same contract.
• Add regression tests proving malformed nested options cannot be persisted.
• Confirm valid existing decision-tree data still loads and the browser advisor initializes successfully.