feat(export): convert evidence files to OpenInference

[drewstone]

What changed

• Adds traces export <input> --out <spans.jsonl> for compact traces.policy_evidence.session JSONL, Sandbox/OpenCode event arrays, and already-OpenInference JSONL.
• Writes HALO-readable OpenInference JSONL through the existing serializeSpans and local redactSpans path.
• Adds traces export --help / traces help export with examples, plus README usage and SDK export docs.

Why

agent-eval normalizes its own run records and trace sources, but issue #4 is about artifacts that skipped that path: compact policy evidence files and raw Sandbox/OpenCode event arrays. This makes traces the file adapter into the one OpenInference output format.

Closes #4.

Verification

• pnpm typecheck
• pnpm test (70/70 tests)
• pnpm build
• git diff --check
• pnpm exec tsx src/cli.ts export --help
• pnpm exec tsx src/cli.ts help export
• CLI smoke: exported a compact policy evidence JSONL file, then checked all 9 required fields (trace_id, span_id, name, start_time, end_time, status, resource, scope, attributes).