chore: archive implement-openai-provider change

[avoidwork]

Description

Archived completed change implement-openai-provider to openspec/changes/archive/2026-05-25-implement-openai-provider/. All 18 tasks complete, all artifacts done, schema: spec-driven.

Type of Change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work in expected)
• Documentation update
• Refactor (no functional changes)
• Performance improvement
• CI / build / tooling

Testing

N/A — pure archive/rename operation with no code changes. Pre-commit hook ran linter, formatter, and tests successfully.

Coverage

• 100% line coverage maintained

Checklist

• npm run lint passes
• Tests pass with 100% line coverage
• No forbidden patterns used
• Conventional Commit style applied