feat(engine): auto-parse script agent JSON stdout into output fields

[PolyphonyRequiem]

Summary

When a script agent's stdout is valid JSON, the parsed fields are now merged into the output dict alongside stdout/stderr/exit_code. This makes parsed fields accessible in route conditions and downstream templates as output.field_name — matching LLM structured output behavior.

Motivation

Script agents are the deterministic workhorses of conductor workflows (P8). They emit structured data via JSON stdout, but currently that data is only accessible as a raw string (output.stdout). Routing on specific fields requires opaque exit codes instead of readable field-based conditions.

Before:

# Must use exit codes for routing
routes:
  - to: planner
    when: "exit_code == 20"  # What does 20 mean?

After:

# Route on parsed JSON fields directly
routes:
  - to: planner
    when: "route == 'planning'"  # Self-documenting

Changes

• src/conductor/engine/workflow.py: After capturing script output, attempt json.loads(stdout). If it parses as a dict, merge fields into output_content (7 lines).
• tests/test_engine/test_workflow.py: End-to-end test verifying JSON parsing, field access, and field-based routing.

Design decisions

• Backward compatible — stdout, stderr, exit_code always present. Non-JSON stdout is unchanged. JSON arrays/scalars are ignored (only objects merged).
• No schema changes — parsed fields are dynamic, not declared. This matches how script output is already untyped.
• Merge semantics — parsed fields could theoretically shadow stdout/stderr/exit_code if a script outputs those as JSON keys. This is intentional: scripts that output {"exit_code": 42} probably want the JSON value to win over the raw field.

Testing

• 75 workflow engine tests pass (including new JSON parsing test)
• Test covers: field parsing, backward compat (stdout still present), field-based routing

[codecov-commenter]

Codecov Report

❌ Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@dbcf20f). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
src/conductor/engine/workflow.py	88.88%	1 Missing ⚠️

Additional details and impacted files

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  Coverage        ?   84.90%           
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  Files           ?       53           
  Lines           ?     7174           
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  Hits            ?     6091           
  Misses          ?     1083           
  Partials        ?        0           

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