docs: clarify contributor development workflow

[Eudes-Crabe]

Summary

• document the editable dev dependency install command in the README Development section
• surface the local ruff, mypy, and pytest commands currently only documented in CLAUDE.md
• add a short Conventional Commits PR-title note for contributors

Verification

• git diff --check -- README.md
• local README content sanity check confirmed the Development section includes:
  • pip install -e ".[dev]"
  • python -m ruff check src/ tests/ --fix
  • python -m ruff format src/ tests/
  • python -m mypy src/
  • python -m pytest tests/
  • Conventional Commits guidance

Duplicate check

GitHub API/search checks at preparation time:

• repo:anthropics/claude-agent-sdk-python is:pr is:open "pip install -e" "dev" → 0 results
• repo:anthropics/claude-agent-sdk-python is:pr is:open "Conventional Commits" README → 0 results
• repo:anthropics/claude-agent-sdk-python is:pr is:open "#966" → 0 results
• A broader search for "python -m ruff check src/ tests" returned existing open PRs touching code areas, but none document the contributor development setup or reference [Docs] Surface dev-deps install + test/lint commands in README (or add CONTRIBUTING.md) #966.