feat(nix): integrate uv2nix for Python dependency management

Replace uv-managed .venv with Nix-managed Python environments using
uv2nix. This provides fully reproducible builds with dependencies
cached in the Nix store, eliminating the need for uv sync in CI.

Key changes:
- Add uv2nix, pyproject-nix, and pyproject-build-systems inputs
- Create devShells for Python 3.11 and 3.13 (default, python311, python313)
- Add build system overrides for pypika (setuptools) and stackone-ai (editables)
- Update CI matrix to use nix develop .#pythonXXX instead of uv sync
- Simplify setup-nix action with gc-max-store-size for cache management
- Add lint-fix alias to justfile

The .venv directory is no longer needed as all dependencies are
managed by Nix. Both Nix and non-Nix users can still use uv run
commands which will work in either environment.

Author: ryoppippi

.github/actions/setup-nix/action.yaml

@@ -12,7 +12,4 @@ runs:
       uses: nix-community/cache-nix-action@b426b118b6dc86d6952988d396aa7c6b09776d08 # v7
       with:
         primary-key: nix-${{ runner.os }}
-
-    - name: Load Nix development environment
-      shell: bash
-      run: nix develop --command true
+        gc-max-store-size: 4G

.github/workflows/ci.yaml

@@ -34,12 +34,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ["3.11", "3.13"]
-        include:
-          - python-version: "3.11"
-            sync-extras: "--all-extras"
-          - python-version: "3.13"
-            sync-extras: "--all-extras"
+        python-version: ["python311", "python313"]
     env:
       STACKONE_API_KEY: ${{ secrets.STACKONE_API_KEY }}
       OPENAI_API_KEY: ${{ secrets.OPENAI_API_KEY }}
@@ -52,17 +47,14 @@ jobs:
       - name: Setup Nix
         uses: ./.github/actions/setup-nix
 
-      - name: Install dependencies
-        run: nix develop --command just install ${{ matrix.sync-extras }}
-
       - name: Run Lint
-        run: nix develop --command just lint
+        run: nix develop .#${{ matrix.python-version }} --command just lint
 
       - name: Run Ty
-        run: nix develop --command just ty
+        run: nix develop .#${{ matrix.python-version }} --command just ty
 
       - name: Run Tests
-        run: nix develop --command just test
+        run: nix develop .#${{ matrix.python-version }} --command just test
 
   coverage:
     runs-on: ubuntu-latest
@@ -79,9 +71,6 @@ jobs:
       - name: Setup Nix
         uses: ./.github/actions/setup-nix
 
-      - name: Install dependencies
-        run: nix develop --command just install --all-extras
-
       - name: Run Tests with Coverage
         run: nix develop --command just coverage