dsh-o11y-plugin: Unified plugin-level observability for deepseek-harness

dsh-o11y-plugin is an observability plugin designed specifically for deepseek-harness (dsh), aiming to unify telemetry management at the plugin level. By registering global OpenTelemetry providers, it enables plugins based on the standard @opentelemetry/api to conveniently export traces, metrics, and logs, while bridging DSH session telemetry into OTel logs. The plugin supports both headless and web profiles, exporting data via OTLP/HTTP endpoints; if no endpoint is configured or it is unreachable, data will be silently discarded. Stateless by design, the plugin requires no data cleanup upon removal, does not write to files or store credentials, and ensures telemetry data resides in memory only before export.