ci: optimize test workflows — reduce jobs, share venv via artifact

- Restructure tests.yml: install once per Python version, share .venv
  via artifact instead of 32 independent installs
- Reduce test groups from 8 to 4 (tests only take ~60s per group)
- Only test Python 3.12+3.13 on PRs; full matrix on push to main
- Switch all workflows from manual actions/cache to setup-uv built-in
  caching, eliminating cache race conditions
- Add --frozen flag to uv sync for deterministic CI installs
- Re-enable duration-based test splitting with least_duration algorithm
  (was disabled due to a bug in the path filter)
- Fix update-test-durations path filter (tests/**/*.py never matched
  actual test dirs under lib/)
- Add concurrency group with cancel-in-progress for PR runs
- Add gate jobs to satisfy existing branch protection required checks

.github/workflows/linter.yml

@@ -18,27 +18,15 @@ jobs:
       - name: Fetch Target Branch
         run: git fetch origin $TARGET_BRANCH --depth=1
 
-      - name: Restore global uv cache
-        id: cache-restore
-        uses: actions/cache/restore@v4
-        with:
-          path: |
-            ~/.cache/uv
-            ~/.local/share/uv
-            .venv
-          key: uv-main-py3.11-${{ hashFiles('uv.lock') }}
-          restore-keys: |
-            uv-main-py3.11-
-
       - name: Install uv
         uses: astral-sh/setup-uv@v6
         with:
           version: "0.8.4"
           python-version: "3.11"
-          enable-cache: false
+          enable-cache: true
 
       - name: Install dependencies
-        run: uv sync --all-groups --all-extras --no-install-project
+        run: uv sync --all-groups --all-extras --frozen --no-install-project
 
       - name: Get Changed Python Files
         id: changed-files
@@ -57,13 +45,3 @@ jobs:
            | grep -v 'src/crewai/cli/templates/' \
            | grep -v '/tests/' \
            | xargs -I{} uv run ruff check "{}"
-
-      - name: Save uv caches
-        if: steps.cache-restore.outputs.cache-hit != 'true'
-        uses: actions/cache/save@v4
-        with:
-          path: |
-            ~/.cache/uv
-            ~/.local/share/uv
-            .venv
-          key: uv-main-py3.11-${{ hashFiles('uv.lock') }}