chore(deps): add ruff to dev dependency group

[iksnerd]

Description

CONTRIBUTING.md (lines 73-79) tells contributors to run:

```bash
uv run ruff format .
uv run ruff check --fix .
```

before every commit. However, ruff is not listed under [dependency-groups].dev in pyproject.toml, so uv sync --dev does not install it. The commands above fail on a fresh checkout with:

```
error: Failed to spawn: `ruff`
Caused by: No such file or directory (os error 2)
```

The current workaround is uvx ruff …, which installs ruff into a transient location on every invocation. This PR adds ruff to the dev group so the documented workflow works as written.

Changes

• Dependency update (`uv lock`)
• Documentation / Tutorial update
• Refactoring (no logic change)

Files changed:

```
pyproject.toml | 1 +
uv.lock | 27 +++++++++++++++++++++++++++
2 files changed, 28 insertions(+)
```

pyproject.toml — adds \"ruff>=0.6\" to the dev group. Lower bound is intentionally loose (>=0.6) so uv's resolver can pick whatever is compatible with the rest of the dev tree; tighter pinning would only be worth it if a particular ruff version became required.

uv.lock — regenerated by uv sync --group dev. Resolves ruff 0.15.14 at the time of this PR.

Verification

```bash
uv sync --group dev # → Installed ruff==0.15.14
uv run ruff --version # → ruff 0.15.14
uv run ruff check src/servers/utilities/main.py # → All checks passed!
```

The documented workflow now works on a fresh uv sync.

Checklist

• I have signed off my commits (DCO).