chore: switch CI to uv, remove uv.lock from gitignore (#885)

* chore: declare pytest as a uv dev dependency

The contributing guide currently tells contributors `pip install pytest`
as a separate step, and CI does the same. Move pytest into PEP 735
`[dependency-groups]` so it's declared in pyproject.toml and `uv sync`
installs it by default (no `--with` workaround, no separate install
line). Update CI to use astral-sh/setup-uv + `uv sync` + `uv run pytest`,
and refresh the Contributing section of the README to match.

`[dependency-groups]` is the right home (vs `[project.optional-dependencies]`)
because pytest is dev-only and shouldn't appear in the published wheel's
optional features list alongside things like `pdf` or `mcp`.

* remove uv.lock from gitignore

.github/workflows/ci.yml

@@ -15,23 +15,20 @@ jobs:
         python-version: ["3.10", "3.12"]
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
-      - name: Set up Python ${{ matrix.python-version }}
-        uses: actions/setup-python@v5
+      - name: Install uv
+        uses: astral-sh/setup-uv@v8.1.0
         with:
           python-version: ${{ matrix.python-version }}
 
       - name: Install dependencies
-        run: |
-          pip install -e ".[mcp,pdf,watch,sql]"
-          pip install pytest
+        run: uv sync --extra mcp --extra pdf --extra watch --extra sql
 
       - name: Run tests
-        run: |
-          python -m pytest tests/ -q --tb=short
+        run: uv run pytest tests/ -q --tb=short
 
       - name: Verify install works end-to-end
         run: |
-          graphify --help
-          graphify install
+          uv run graphify --help
+          uv run graphify install