fix(observability): sanitize LangSmith trace payloads

[ngoclam9415]

Summary

• sanitize LangSmith trace inputs/outputs before SDK serialization so cyclic Dana timelines are logged as compact summaries
• add regression coverage for cyclic Timeline payloads and JSON-safe trace processors
• add a repo-local dana-cross-repo-debug skill for reproducing console-bug issues through tmux, backend API, and the Playground

Test plan

• uv run ruff check dana/common/observable.py dana/common/trace_sanitizer.py tests/unit/test_observable_langsmith_dispatch.py
• uv run python -m pytest tests/unit/test_observable_langsmith_dispatch.py -q
• python -m json.tool .claude/skills/dana-cross-repo-debug/evals/evals.json
• console-bug backend agent.ask repro returned HTTP 200 with no LangSmith recursion crash after local dana-runtime binding