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

docs/Tuning-Guide.md

@@ -645,7 +645,7 @@ How long a new head's review waits while its checks are pending. A review
 judges the CI result (R1), and waiting keeps it off a GPU that CI is using
 (the `nisse` project's end-to-end tests). Finished checks, passed or failed,
 end the wait; after this long it reviews anyway and the CI result reads as
-pending.
+pending. 0 never waits.
 
 - **Too low:** reviews run beside CI, slower on a shared GPU, without its result.
 - **Too high:** a stuck runner delays reviews that long.
@@ -656,7 +656,8 @@ pending.
 
 How long a new head with no checks at all waits for some to appear: Gitea
 Actions reports a push's checks a moment after it. Past this, a repository
-without CI is reviewed at once.
+without CI is reviewed at once. 0 reviews such a head at once (the live
+review test, whose fresh repository has no CI).
 
 - **Too low:** a review starts before CI has reported, and runs beside it.
 - **Too high:** repositories without CI wait that long for every review.