Update OTEL examples with correct trace() usage pattern

Corrects the documentation and code to reflect the proper usage of trace():
- trace() should be used in workflows when using custom_span() (patterns 2 & 3)
- Never use trace() in client code
- Pattern 1 (basic): No trace() needed - plugin creates root trace automatically
- Pattern 2 (custom spans): trace() + custom_span() in workflow
- Pattern 3 (direct API): trace() + custom_span() + OTEL tracer in workflow

Updates:
- README with clearer pattern explanations and examples
- Direct API workflow to use trace() + custom_span() pattern
- All docstrings to document the correct patterns
- Troubleshooting section with pattern-specific guidance

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: jssmith