ci: three integration shards, not two

Two shards cleared in about fourteen minutes on a quiet day, but with six
pull requests running side by side the larger shard hit the same 25-minute
guard the split was meant to escape. Three shards keep the biggest slice
around sixty files, which clears the guard with real headroom on the
slowest runner observed. The gate job is unchanged.

.github/workflows/integration.yml

@@ -10,21 +10,21 @@ permissions:
   contents: read
 
 jobs:
-  # The suite outgrew a single runner: 128 files took ~13 minutes when the
-  # 25-minute guard was written, and the v1.37 sharing work took it to 173
-  # files and past 25 minutes of wall clock, so every run died at the timeout
-  # with every test to that point green. The guard's own rule applies — split,
-  # don't raise — so the files run as two vitest shards. Each shard boots its
-  # own Postgres testcontainer; the timeout stays a hang detector, sized so a
-  # healthy half-suite clears it on a slow runner.
+  # The suite outgrew a single runner, and then a half-suite outgrew a slow
+  # one: two shards cleared in ~14 minutes on a quiet day, but with six pull
+  # requests running side by side the larger shard hit the same 25-minute
+  # guard the split was meant to escape. Three shards keep the biggest slice
+  # around 60 files, which clears the guard with real headroom even on the
+  # slowest runner observed. Each shard boots its own Postgres testcontainer;
+  # the timeout stays a hang detector, not a performance budget.
   shard:
     runs-on: ubuntu-latest
     timeout-minutes: 25
     strategy:
       fail-fast: false
       matrix:
-        shard: [1, 2]
-    name: integration shard ${{ matrix.shard }}/2
+        shard: [1, 2, 3]
+    name: integration shard ${{ matrix.shard }}/3
     steps:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
 
@@ -44,7 +44,7 @@ jobs:
       # vitest a stray positional argument and the shard flag never applies —
       # both "shards" then run the full suite. Proven by a paired local run
       # before this workflow shipped.
-      - run: pnpm exec vitest run --config vitest.integration.config.mts --shard=${{ matrix.shard }}/2
+      - run: pnpm exec vitest run --config vitest.integration.config.mts --shard=${{ matrix.shard }}/3
         env:
           # The integration suite boots a Postgres testcontainer and points
           # Prisma at it via DATABASE_URL set inside `tests/integration/setup.ts`.