Tuning: the CI waits may be zero

The live review test sets ci_grace_s: 0, which the shared tuning parser
refused (every number had to be positive). A field now opts in to zero
with metadata {"zero": True}; ci_wait_s and ci_grace_s do.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01J5Utyfzw7gF7UhjRDEtjTk

nisse-core/tests/test_shared_config.py

@@ -19,6 +19,7 @@ from nisse_core.config import (
 class Review:
     max_rounds: int = 3
     wait_s: float = 60.0
+    grace_s: float = field(default=5.0, metadata={"zero": True})
 
 
 @dataclass(frozen=True)
@@ -45,6 +46,17 @@ def test_an_agents_own_tuning_sections() -> None:
     ]
 
 
+def test_a_field_may_opt_in_to_zero() -> None:
+    errors: list[str] = []
+    tuning = parse_tuning({"review": {"grace_s": 0}}, Tuning, errors)
+    assert tuning.review.grace_s == 0 and errors == []
+    parse_tuning({"review": {"grace_s": -1, "wait_s": 0}}, Tuning, errors)
+    assert errors == [
+        "tuning.review.grace_s: must be zero or more",
+        "tuning.review.wait_s: must be a positive number",
+    ]
+
+
 def test_references_and_people() -> None:
     errors: list[str] = []
     env = {"TOKEN": "s3cret"}