A repository per test run instead of the shared sandbox

Every live test that touches a repository gets one of its own from
e2e_support.fresh_repo, made by the test operator as an operator makes a
project's (D4, D8): seeded from tests/integration/seed/ (the sandbox's
package, without past runs' leftovers), the agent's account a collaborator
with write access, main protected (pushes only by the owner, an approval to
merge), and a deploy key generated for the run. It is deleted afterwards
(NISSE_E2E_KEEP_REPOS keeps it); scripts/e2e-cleanup.sh deletes leftovers.

- Both e2e fixtures use it; per-run base branches are gone. The merge
  helper approves first, as the protection now requires.
- test_gitea_live uses it, so CI needs NISSE_OPERATOR_TOKEN and no longer
  writes the sandbox deploy key (NISSE_PUSH_KEY).
- A live test of the helper: seeded, shared with the agent, protected
  against it, deleted.
- nisse-test.yaml's project entry is a placeholder; pytest and ruff skip
  the seed.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPQ14L3LikyefCMFBqEbKE

tests/integration/seed/sandbox/mathutils.py

@@ -0,0 +1,16 @@
+"""Math utilities."""
+
+
+def factorial(n: int) -> int:
+    """Return n! for a non-negative integer n.
+
+    Raises TypeError if n is not an int (bool is rejected), ValueError if n < 0.
+    """
+    if isinstance(n, bool) or not isinstance(n, int):
+        raise TypeError(f"n must be an int, got {type(n).__name__}")
+    if n < 0:
+        raise ValueError(f"n must be >= 0, got {n}")
+    result = 1
+    for i in range(2, n + 1):
+        result *= i
+    return result