[v6] Logging & Diagnostics Implementation #1503
Description
Scope: Implement Logger with normal/diagnostics modes + JSON structured output.
Acceptance Criteria:
- Normal run ≤25 log lines for full composite scenario.
- Diagnostics run captures diffs & file lists.
- Secret masking tests pass.
Tasks: - Implement logger levels & masking.
- Integrate into StepRunner & services.
- Add snapshot tests.
Labels: enhancement