M1.2: Trace logger with tests
TraceLogger produces JSONL audit logs per research() call:
- One file per trace_id at ~/.marchwarden/traces/{trace_id}.jsonl
- Each line is a self-contained JSON object (step, action, timestamp, decision)
- Supports arbitrary kwargs (url, content_hash, query, etc.)
- Lazy file handle, flush after each write, context manager support
- read_entries() for replay and testing
15 tests: file creation, step counting, JSONL validity, kwargs,
timestamps, flush behavior, multiple independent traces.
Refs: archeious/marchwarden#1
Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>
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