[Docs] Surface dev-deps install + test/lint commands in README (or add CONTRIBUTING.md)

[aregmii]

Short description

The README's ## Development section covers installing the pre-push git hook (scripts/initial-setup.sh) but does not document the dev-dependency install command (pip install -e ".[dev]") or the manual lint / typecheck / test commands. Those commands exist in CLAUDE.md, which is structured to guide Claude Code agents at runtime, not for human contributors who would naturally look for a CONTRIBUTING.md. Adding the missing pieces in the README or as a standalone CONTRIBUTING.md would allow external contributors to find the workflow without reverse-engineering it from merged PR history.

Steps to reproduce

1. Clone the repo and read README.md from top to bottom.

2. The ## Development section says: run scripts/initial-setup.sh to install git hooks. The hook runs "lint checks" but the manual command equivalents aren't documented.

3. pyproject.toml declares a dev extras group (pytest, pytest-asyncio, mypy, ruff, pytest-cov), but the install command (pip install -e ".[dev]") is not in the README.

4. CLAUDE.md documents the manual commands:

   • python -m ruff check src/ tests/ --fix
   • python -m ruff format src/ tests/
   • python -m mypy src/
   • python -m pytest tests/

   These are not surfaced anywhere a human contributor would naturally look.

5. PR title format (Conventional Commits) is used consistently across recent merged PRs (docs(types): clarify hooks dispatch is concurrent, not sequential #956, fix(types): tighten permission_suggestions type in SDKControlPermissionRequest #955, Export EffortLevel type alias #951, test(transcript_mirror): stabilize eager-flush tests with deterministic wait #933, fix(transport): isolate stderr callback failures, continue reading lines #932) but never documented.

Expected behavior

Either:

• Expand the ## Development section in README.md to include the dev-deps install command, the manual lint/typecheck/test commands, and the PR title convention, OR
• Add a CONTRIBUTING.md at the repo root and link it from the README's ## Development section.

Actual behavior

External contributors must read pyproject.toml (to discover the dev extras group), open CLAUDE.md (to find lint/test commands intended for Claude Code agents), and scan recent merged PRs (to infer the title format) before they can submit a clean PR.

Environment

OS: macOS / Python: 3.12 / claude-agent-sdk: 0.2.82 (latest).

Additional context

The information already exists across the repo; the fix is to consolidate it where contributors will find it. Happy to follow up with a PR adding CONTRIBUTING.md (or expanding the Development section) once the maintainer-preferred shape is confirmed.