docs(readme): document dev setup, local checks, and PR conventions

[aregmii]

Summary

Closes #966.

Expands the ## Development section so external contributors can find the dev workflow without reverse-engineering it from CLAUDE.md, pyproject.toml, and merged PR history.

Added (addresses every bullet from #966):

• pip install -e ".[dev]" for the dev dependencies (pytest, pytest-asyncio, pytest-cov, mypy, ruff)
• The manual ruff / mypy / pytest commands that the pre-push hook wraps
• The Conventional Commits PR title convention used by recent merged PRs

Also kept (the original commit on this branch):

• The examples/hooks.py reference now uses a markdown link, matching the format of every other example reference in the README

Diff (README only, +36 / -3)

 ## Development

-If you're contributing to this project, run the initial setup script to install git hooks:
+### Setting up a development environment
+
+Install the SDK in editable mode along with the dev dependencies (`pytest`, `pytest-asyncio`, `pytest-cov`, `mypy`, `ruff`):
+
+```bash
+pip install -e ".[dev]"
+```
+
+Then install the pre-push git hook so lint checks run before each push (matching CI):

 ```bash
 ./scripts/initial-setup.sh
 ```

-This installs a pre-push hook that runs lint checks before pushing, matching the CI workflow. To skip the hook temporarily, use `git push --no-verify`.
+To skip the hook for a single push, use `git push --no-verify`.
+
+### Running checks locally
+
+The pre-push hook wraps the same commands you can run directly:
+
+```bash
+# Lint and auto-fix
+python -m ruff check src/ tests/ --fix
+
+# Format
+python -m ruff format src/ tests/
+
+# Type-check
+python -m mypy src/
+
+# Tests (all)
+python -m pytest tests/
+
+# Tests (single file)
+python -m pytest tests/test_client.py
+```
+
+### Pull request conventions
+
+PR titles follow [Conventional Commits](https://www.conventionalcommits.org/), e.g. `fix(transport): handle CancelledError`, `docs: update changelog`, `feat(hooks): add SubagentStartHookInput`. Recent merged PRs are good references for tone and scope.

 ### Building Wheels Locally

And the smaller pre-existing change on the same branch:

-For more examples, see examples/hooks.py.
+For more examples, see [examples/hooks.py](examples/hooks.py).

Verification

All four documented commands run cleanly on a fresh venv against current main (Python 3.11.7):

$ python -m venv .venv-verify && source .venv-verify/bin/activate
$ pip install -e ".[dev]"
Successfully installed claude-agent-sdk-0.2.82 mypy-2.1.0 pytest-9.0.3
  pytest-asyncio-1.3.0 pytest-cov-7.1.0 ruff-0.15.13 ... (and transitive deps)

$ python -m ruff check src/ tests/
All checks passed!

$ python -m ruff format --check src/ tests/
54 files already formatted

$ python -m mypy src/
Success: no issues found in 24 source files

$ python -m pytest tests/
750 passed, 5 skipped in 6.40s

Conventional Commits convention verified against the last 8 merged PRs: 7 of 8 follow <type>(scope): subject or <type>: subject (PRs #956, #955, #933, #932, #931, #927, #924, plus #951 as the one outlier).

The PR title format itself follows the convention (docs(readme): ...).

Test plan

Documentation-only change; no source code or behavior modified. The added commands match what CLAUDE.md documents and what pyproject.toml's [dev] extras group declares. Verification log above shows each command succeeds on a clean checkout.