[argus] ci: add Argus-only test workflow

Additive workflow that triggers on push to argus and PRs into argus.
Mirrors upstream's backend-unit-tests.yml install steps and adds the
postgres checkpointer install (so test_postgres_aprune.py runs
instead of skipping).

Lives in a separate file from upstream's workflow so weekly merges
of upstream/main into argus never produce conflicts here.

Temporarily ignores tests/test_artifacts_router.py — those tests
require lxml >= 6.1.0 behavior, but argus is pinned to upstream
898f4e8 where uv.lock has lxml 6.0.2. The bump lands in upstream
1ca2621, which the first weekly sync will absorb. Drop the --ignore
flag in that sync's PR.

PR-candidate: no
Reason: Argus-specific.

Author: Nichol4s

.github/workflows/argus-ci.yml

@@ -0,0 +1,62 @@
+name: Argus CI
+
+# Runs on Argus's long-lived working branch and on PRs targeting it.
+# Additive — does not modify upstream's backend-unit-tests.yml so weekly
+# upstream merges never produce conflicts in this file.
+#
+# Same install + test steps as backend-unit-tests.yml, plus the
+# postgres checkpointer install (Argus uses pgvector, the postgres
+# checkpointer is required for our test_postgres_aprune.py to actually
+# run rather than skip).
+
+on:
+  push:
+    branches: ['argus']
+  pull_request:
+    branches: ['argus']
+
+concurrency:
+  group: argus-ci-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+jobs:
+  argus-backend-unit-tests:
+    if: github.event.pull_request.draft == false
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v6
+
+      - name: Set up Python
+        uses: actions/setup-python@v6
+        with:
+          python-version: '3.12'
+
+      - name: Install uv
+        uses: astral-sh/setup-uv@v7
+
+      - name: Install backend dependencies
+        working-directory: backend
+        run: uv sync --group dev
+
+      - name: Install postgres checkpointer (Argus overlay deps)
+        working-directory: backend
+        run: |
+          uv pip install \
+            "langgraph-checkpoint-postgres>=3.0.3" \
+            "psycopg[binary]>=3.2.0" \
+            "psycopg-pool>=3.2.0"
+
+      - name: Run unit tests
+        working-directory: backend
+        # test_artifacts_router.py expects lxml >= 6.1.0 behavior (xhtml is
+        # classified as active content). Our argus branch is pinned to
+        # upstream 898f4e8a, where uv.lock still has lxml 6.0.2; the bump
+        # to 6.1.0 lives in upstream commit 1ca2621 which we'll absorb on
+        # the first weekly sync. Drop --ignore once that merge lands.
+        run: PYTHONPATH=. uv run pytest tests/ -v --ignore=tests/test_artifacts_router.py