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

## 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 (#956, #955, #951, #933, #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.

Read README.md's ## Development section, then compare the contributor-facing details in pyproject.toml and CLAUDE.md. Consolidate the documented dev-dependency install, ruff, mypy, and pytest commands, plus the PR title convention, in the maintainer-preferred README.md or a new root CONTRIBUTING.md linked from the README. Done means an external contributor can find the complete workflow without consulting merged PRs.

Written by the indexing model from the issue text.