Empty result with --permission plan mode

[landigf]

Bug description

When running a pipeline with --permission plan, Claude executes for several minutes but returns an empty result. The step fails with:

[claudeflow] WARNING: Claude returned empty result. This usually means it wrote to a file instead of returning text.
✗ Step "code-audit" failed after 1 attempts: Output schema validation failed for step "code-audit"

Steps to reproduce

1. Create a pipeline with tool-enabled steps (tools: [Read, Glob, Grep]) and an output schema
2. Run with --permission plan
3. Claude works for minutes (241s in my case) but returns empty

Expected behavior

Claude should return its analysis as text, which claudeflow parses against the output schema.

Actual behavior

Empty result despite 4 minutes of execution. The runtime can't capture Claude's output.

Environment

• claudeflow 0.1.0
• Claude CLI (claude code)
• macOS

Notes

• The warning suggests "Adding --system-prompt flag should prevent this" but it's unclear how to pass this via CLI or YAML
• Even with explicit system: prompts in YAML saying "rispondi SEMPRE con testo, MAI scrivere file", the issue persists
• This might be a fundamental incompatibility between plan mode and --output-format json