Speed up CI: switch lint to ruff-action, drop broken pip cache

- lint job now uses astral-sh/ruff-action instead of provisioning a
  full Python toolchain via setup-python just to pip install ruff —
  the action just downloads the standalone Ruff binary.
- build job's cache: pip was hanging on every run: the Gitea runner
  has no reachable Actions-cache backend, so setup-python's cache
  restore step was timing out on a connect (ETIMEDOUT) before
  falling back and continuing anyway. Removed since deps here are
  tiny and a cold install costs seconds, not minutes.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -11,13 +11,12 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
-      - uses: actions/setup-python@v5
+      - uses: astral-sh/ruff-action@v3
         with:
-          python-version: "3.13"
-          cache: pip
-      - run: pip install ruff
-      - run: ruff check src/
-      - run: ruff format --check src/
+          args: check src/
+      - uses: astral-sh/ruff-action@v3
+        with:
+          args: format --check src/
 
   build:
     runs-on: ubuntu-latest
@@ -26,7 +25,6 @@ jobs:
       - uses: actions/setup-python@v5
         with:
           python-version: "3.13"
-          cache: pip
       - run: pip install -r requirements.txt
       - name: Verify imports
         run: python -c "from src.server import mcp; print('OK:', mcp.name)"