fix: forward modelRegistry and model to workflow subagents

[Paul-Yuchao-Dong]

Problem

Subagents spawned by the workflow tool fail silently -- they return null or empty strings even for trivial tasks like echo "test". The workflow tool reports "N agent(s) completed" but produces no usable output.

Root Cause

The WorkflowAgent creates subagent sessions via createAgentSession() with SettingsManager.inMemory() but never passes modelRegistry or model. Without modelRegistry, the subagent session cannot authenticate to the LLM provider -- session.prompt() either fails silently or returns empty responses.

By contrast, the Agent tool from @tintinweb/pi-subagents works correctly because it explicitly passes:

modelRegistry: ctx.modelRegistry,
model: ctx.model,

Fix

In src/workflow-tool.ts, pass modelRegistry and model from the tool execution context into the runWorkflow options as a session property. This flows through:

workflow-tool.ts execute(ctx) -> runWorkflow(session) -> WorkflowAgent(session) -> createAgentSession({...session})

Verification

Tested with a minimal workflow (echo test with structured output schema) -- subagents now produce correct JSON output instead of returning null. A 5-agent inspection workflow on a real project also completed successfully with all agents returning structured results.