Add OpenTelemetry JSON trace exporter

[weich97]

Scope

Export an existing TradeArena trajectory JSON to an OpenTelemetry-style JSON span file.

Suggested files

• src/tradearena/
• tests/
• docs/observability.md

Acceptance criteria

• Exporter accepts an existing trajectory file and does not rerun experiments.
• Raw provider text is excluded by default.
• Test checks span names, parent IDs, and redaction fields.

Validation

python -m pytest tests/test_observability_export.py -q