feat(langfuse): migrate observability integration

[Yangtze-Seventh]

Summary

• migrate the complete EverOS × Langfuse integration from EverOS/examples/langfuse into plugins/langfuse
• preserve the live OpenTelemetry demo, local trace recorder, 237-span/60-trace fixture, and replay utility
• make replay fixture discovery independent of the current working directory
• add pinned replay dependencies, offline contract tests, a full local replay round trip, and path-scoped CI
• document Langfuse as an observability integration in the repository catalog

EverOS keeps its vendor-neutral OpenTelemetry runtime and Langfuse-compatible attributes/scores; only the ecosystem-facing example is moved.

Verification

• ruff check demo.py replay.py record_trace.py tests
• python -m py_compile demo.py replay.py record_trace.py
• python replay.py --help
• python record_trace.py --help
• python -m unittest discover -s tests -v
• end-to-end replay: 237 spans and 47 scores sent to and re-recorded by the local compatible sink
• migrated demo.py, record_trace.py, recorded_trace.json, and .gitignore byte-for-byte compared with the EverOS source
• staged secret-pattern scan: no credentials detected