ci: optimize Test Suite workflow (safe subset)

Re-applies the safe portion of c01f8599 after the revert in 2e8e674e.
Drops the uv cache which broke last time because uv.lock is gitignored
in this repo, and keeps lint as a step inside the test job (matching
the pre-c01f8599 layout).

What's added (all metadata-only, no external dependencies):
- concurrency: cancel superseded runs on the same ref
- permissions: tighten GITHUB_TOKEN to contents: read
- timeout-minutes: 20 to bound runaway jobs
- fail-fast: false so all matrix combinations surface failures
- matrix conditional: PRs run Linux x {3.11, 3.14} for fast feedback;
  push to main/nightly still runs the full 2-OS x 4-Python matrix

What's intentionally NOT added (each removed for a reason):
- uv cache: depends on uv.lock which is gitignored
- separate lint job: kept inline as a step, matches original
- workflow_dispatch / paths-ignore: scope creep, not needed now

All jobs continue to run on standard GitHub-hosted runners
(ubuntu-latest, windows-latest), keeping CI within the free tier.

Co-authored-by: Cursor <cursoragent@cursor.com>

.github/workflows/ci.yml

@@ -6,13 +6,22 @@ on:
   pull_request:
     branches: [main, nightly]
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
 jobs:
   test:
     runs-on: ${{ matrix.os }}
+    timeout-minutes: 20
     strategy:
+      fail-fast: false
       matrix:
-        os: [ubuntu-latest, windows-latest]
-        python-version: ["3.11", "3.12", "3.13", "3.14"]
+        os: ${{ github.event_name == 'pull_request' && fromJSON('["ubuntu-latest"]') || fromJSON('["ubuntu-latest","windows-latest"]') }}
+        python-version: ${{ github.event_name == 'pull_request' && fromJSON('["3.11","3.14"]') || fromJSON('["3.11","3.12","3.13","3.14"]') }}
 
     steps:
       - uses: actions/checkout@v4