Add otel skill for Copilot Chat OpenTelemetry instrumentation

Adds .github/skills/otel/SKILL.md describing:
- The two source-of-truth docs (agent_monitoring.md and agent_monitoring_arch.md)
- The four agent execution paths and their OTel strategies
- Canonical file map for all OTel code
- Service layer (NoopOTelService / NodeOTelService / InMemoryOTelService)
- Span / metric / event conventions, content-capture gating, debug-panel vs OTLP isolation
- Configuration surface invariants (settings + env vars + agent env-var translation)
- Procedure checklists for adding spans / metrics / events / new agent surfaces
- Validation commands and known risks/anti-patterns

Mandates that the two monitoring docs stay in sync with code changes.