chore(langfuse): move integration to plugins repository - #418

Summary

• remove the ecosystem-facing examples/langfuse directory after migrating it to EverMind-AI/plugins/langfuse
• keep EverOS native OpenTelemetry tracing, Langfuse-compatible attributes, recall-score delivery, configuration, and all core tests unchanged
• preserve historical CHANGELOG entries and repository topics

Companion migration: EverMind-AI/Plugins#10

Scope verification

• the diff deletes exactly the six tracked files under examples/langfuse
• no source, dependency, configuration, test, README, or workflow file changes
• no remaining live references to examples/langfuse outside historical CHANGELOG entries

Tests

• PYTHONPATH=src python -m pytest -q tests/unit/test_config/test_observability_settings.py tests/unit/test_core/test_observability tests/unit/test_core/test_lifespan/test_tracing_lifespan.py tests/unit/test_service/test_memorize_span.py
• result: 53 passed