create-pr: auto-complete + draft warning should fail or skip auto-complete

[jamesadevine]

Problem

The operator gets a log warning but the PR is created without auto-complete, which may surprise them.

Suggested Fix

Either:

1. Return ExecutionResult::failure when both are set (strict — forces operator to fix config)
2. Skip the auto-complete API call when draft: true (lenient — avoids silent failure)
3. Add compile-time validation in the compiler that rejects draft: true + auto-complete: true

Option 2 is probably the best UX — the warning stays, but no pointless API call is made.

Context

This became relevant when draft: true was made the default in PR #155. Existing pipelines with auto-complete: true that don't explicitly set draft: false will silently lose auto-complete behavior.

Identified in PR #155 review.