Page to the end when searching for an existing PR

find_pull_request read only the newest 50 per state, so once a repo
passed that many pull requests a "not found" would mean "not in the
newest page" — and the A3 reconciler would read that as "the PR was
never opened" and open a duplicate. Exactly the side effect the
intent log exists to prevent.

It now pages until a short page proves the end, and raises rather
than answering "absent" from an incomplete search; the reconciler
turns that into unverifiable and gates to the human.

Found while wiring CI: every CI run opens a PR in the sandbox, so the
threshold would have been crossed in weeks rather than never.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

nisse-dev/src/nisse_dev/gitea.py

@@ -20,6 +20,9 @@ import httpx
 from nisse_dev.reconcile import Reconciler
 from nisse_dev.state import Intent
 
+PR_PAGE_SIZE = 50
+MAX_PR_PAGES = 40  # 2000 PRs; beyond that, say so rather than guess
+
 
 class GiteaError(RuntimeError):
     pass
@@ -92,16 +95,31 @@ class GiteaClient:
 
     def find_pull_request(self, repo: str, head: str) -> PullRequest | None:
         """External truth for A3: did the PR for this branch already get
-        opened? Checks closed PRs too — a crash after opening must not be
-        answered with "no PR is open, so open another"."""
-        for state in ("open", "closed"):
+        opened? Includes closed PRs — a crash after opening must not be
+        answered with "no PR is open, so open another".
+
+        Pages to the end rather than sampling the first page: a "not found"
+        that only means "not in the newest 50" would licence a duplicate PR.
+        If the history is longer than this walk, raise instead of guessing —
+        the reconciler turns that into "unverifiable" and gates to the human.
+        """
+        for page in range(1, MAX_PR_PAGES + 1):
             data = self._request(
-                "GET", f"/repos/{repo}/pulls", params={"state": state, "limit": 50}
+                "GET",
+                f"/repos/{repo}/pulls",
+                params={"state": "all", "limit": PR_PAGE_SIZE, "page": page},
             )
-            for item in data or []:
+            items = data or []
+            for item in items:
                 if (item.get("head") or {}).get("ref") == head:
                     return _pull_request(item)
-        return None
+            if len(items) < PR_PAGE_SIZE:
+                return None  # reached the end: definitively absent
+        raise GiteaError(
+            f"searched {MAX_PR_PAGES * PR_PAGE_SIZE} pull requests in {repo} "
+            f"without reaching the end; cannot confirm whether {head!r} was "
+            "already opened"
+        )
 
     def list_comments(
         self, repo: str, index: int, since: str | None = None