refactor: extract shared _build_prompt and _derive_artifacts helpers out of provider adapters

[Ryan-Atkinson87]

Background

PRs #224 (CodexAdapter) and #225 (GeminiAdapter) each contain identical _build_prompt and _derive_artifacts module-level functions. ClaudeAdapter also contains its own _derive_artifacts (at claude.py:317). Any future ProviderAdapter implementation would need to duplicate them again.

Work

• Create orchestrator-api/app/providers/utils.py (or subprocess_helpers.py) exporting:
  • build_prompt(prompt: str, context_files: list[Path]) -> str
  • derive_artifacts(repo: GitRepo, head_before: str) -> Coroutine[..., list[str]]
• Update claude.py, codex.py, and gemini.py to import from the shared module and delete the local copies.
• No behaviour change — pure refactor.

Acceptance criteria

• Single implementation of each helper, imported by all three adapters
• All existing adapter tests still pass (uv run pytest)
• ruff check and pyright clean