Release v1.9.0

What changed

• Introduced src/ai_prompt_runner/api.py as a thin facade over the existing
  execution pipeline (create_provider + PromptRunner).
• Exported the public API from ai_prompt_runner/init.py for stable imports.
• Added dedicated unit tests for library usage:
  • success path
  • system prompt forwarding
  • provider/configuration failure handling
• Updated documentation for library usage and architecture boundary:
  • README.md
  • docs/architecture.md

Why it matters

• Enables clean Python integration while preserving the CLI-first product design.
• Reuses the same runtime path and normalized payload contract as CLI execution.
• Improves embeddability for automation and application-level usage without adding
  orchestration complexity.

Stability Statement

• v1.9.0 is a stable feature release.
• No CLI breaking changes.
• No output JSON schema/contract changes.
• Stateless execution model remains unchanged.

Scope Clarification

• No conversational state or multi-turn behavior.
• No agent/orchestration features.
• No provider factory redesign.
• No breaking changes to existing contracts.

GitHub Pull Requests Included

• Feature/v1.9 library api by @damienSavoldelli in #46
• release: prepare v1.9.0 by @damienSavoldelli in #47

Full Changelog: v1.8.0...v1.9.0