feat(ci): use 6-way slicing based on benchmark results

Benchmarked 4/5/6/7/8 slices with LPT duration-balanced distribution:
- 4 slices: 4.8m wall, 135s spread
- 5 slices: 3.4m wall, 46s spread
- 6 slices: 3.3m wall, 26s spread ← optimal
- 7 slices: 3.9m wall, 109s spread
- 8 slices: 3.7m wall, 96s spread

6 slices is the sweet spot: lowest wall time, tightest spread.
7+ gets slower due to per-slice startup overhead dominating.

Also removes benchmark branch markers from save-durations condition.

.github/workflows/tests.yml

@@ -27,7 +27,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        slice: [1, 2, 3, 4]
+        slice: [1, 2, 3, 4, 5, 6]
     steps:
       - name: Checkout code
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd  # v6.0.2
@@ -65,7 +65,7 @@ jobs:
           source .venv/bin/activate
           uv pip install -e ".[all,dev]"
 
-      - name: Run tests (slice ${{ matrix.slice }}/4)
+      - name: Run tests (slice ${{ matrix.slice }}/6)
         # Per-file isolation via scripts/run_tests_parallel.py: discovers
         # every test_*.py file under tests/ (excluding integration/ + e2e/),
         # then runs `python -m pytest <file>` in a freshly-spawned subprocess
@@ -84,14 +84,14 @@ jobs:
         # fix. ThreadPoolExecutor + subprocess.run is ~60 lines and does
         # the job with cleaner semantics.
         #
-        # Matrix slicing (--slice I/N): files are distributed across 4
+        # Matrix slicing (--slice I/N): files are distributed across 6
         # jobs by cached duration (LPT algorithm) so each job gets
         # roughly equal wall time. Without a cache, files default to 2s
         # estimate and get split roughly evenly by count — still correct,
         # just not perfectly balanced.
         run: |
           source .venv/bin/activate
-          python scripts/run_tests_parallel.py --slice ${{ matrix.slice }}/4
+          python scripts/run_tests_parallel.py --slice ${{ matrix.slice }}/6
         env:
           # Ensure tests don't accidentally call real APIs
           OPENROUTER_API_KEY: ""
@@ -109,7 +109,7 @@ jobs:
   # (including PRs) get balanced slicing.
   save-durations:
     needs: test
-    if: always() && (github.ref == 'refs/heads/main' || startsWith(github.ref, 'refs/heads/ethie/slice-test-'))
+    if: always() && github.ref == 'refs/heads/main'
     runs-on: ubuntu-latest
     steps:
       - name: Download all slice durations