docs(examples): add Langfuse (OpenTelemetry) integration example

[dani1005]

Adds examples/langfuse/ — a thin OpenTelemetry wrapper that traces EverOS memory operations (add / flush+extract / search / reflection) into Langfuse, with recall quality pushed as Langfuse scores.

• Pure OTel SDK, no Langfuse package dependency; works with any OTLP backend.
• Runs with no server via a built-in mock, or against a real EverOS server (EVEROS_BASE_URL). Verified end-to-end (traces + scores render in Langfuse).
• Additive only, no changes to EverOS core.
• Referenced by the upcoming Langfuse docs integration cookbook.

Native, opt-in instrumentation in core is a planned follow-up.

[dani1005]

Heads-up for reviewers on structure:

This introduces a new top-level examples/ directory (it didn't exist before). The intent is for it to be the home for runnable integration / usage examples, one per subfolder — this PR adds examples/langfuse/, and future ones would sit alongside (e.g. examples/<integration>/).

Why examples/ rather than alternatives:

• It's runnable sample code users copy/adapt, not a shipped module — so examples/ fits better than integrations/ (which implies a first-party, packaged integration).
• It's code + a runnable demo, so examples/ reads more naturally than nesting under docs/.

This is additive only — no changes to existing code or core. Native, opt-in instrumentation inside core is a planned follow-up.

If you'd prefer a different home for this (e.g. docs/integrations/ or integrations/), happy to rename before merge — just let me know.