nisse-review: one unlistable repository holds up no other

Found at bring-up: an empty private repository's /pulls answered 404, and
intake stopped there, so the whole round failed and nothing was reviewed.
A repository whose PRs can't be listed is now logged (intake.repo_failed)
and skipped for the round; its items stay as they are.

Also, from nisse-review's review of this PR: a two-page test pins the
search listing the patterns rest on.

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

nisse-review/tests/review_fakes.py

@@ -72,6 +72,7 @@ class FakeGitea:
     down: bool = False
     # Readable but not given (public, no grant): missing from /user/repos.
     public: set[str] = field(default_factory=set)
+    unlisted: set[str] = field(default_factory=set)  # its PRs can't be listed
 
     def _up(self) -> None:
         if self.down:
@@ -99,6 +100,8 @@ class FakeGitea:
 
     def open_pull_requests(self, repo: str) -> list[PullRequest]:
         self._up()
+        if repo in self.unlisted:
+            raise GiteaNotFound(f"gitea GET /repos/{repo}/pulls returned 404")
         return list(self.repos[repo])
 
     def pull_request(self, repo: str, index: int) -> PullRequest: