TEMP: failing test to confirm all-green gates the matrix

Reverts the fault injection and adds a test that always fails; all-green
must go red on this commit. Will be dropped from this branch.

No-Verification-Needed: temporary validation scaffolding, dropped before merge

Author: maxisbey

.github/workflows/shared.yml

@@ -82,7 +82,6 @@ jobs:
         with:
           enable-cache: true
           version: 0.9.5
-          manifest-file: https://example.invalid/uv.ndjson
 
       - name: Install uv (retry)
         if: steps.setup-uv.outcome == 'failure'

tests/test_tempgate.py

@@ -0,0 +1,2 @@
+def test_ci_gate_validation() -> None:
+    raise AssertionError("temporary: proving all-green gates the matrix")