Add OpenTelemetry export for LLM and tool operations

[bug-ops]

Epic: #366

Description

Add optional OpenTelemetry OTLP export for traces and metrics, enabling Prometheus/Grafana integration.

Design

[observability]
exporter = "otlp"  # "none" | "otlp"
endpoint = "http://localhost:4317"

Tasks

• Add opentelemetry, opentelemetry_sdk, opentelemetry-otlp deps (feature-gated otel)
• tracing-opentelemetry layer in subscriber stack
• LLM calls: span with model, prompt_tokens, completion_tokens, latency_ms attributes
• Tool executions: span with tool_name, result, duration_ms
• Memory operations: span with operation (recall/remember), result_count
• Config section [observability] with exporter selection
• Graceful shutdown flushes pending spans