Say every PR needs a tracking issue, and test two more lookup paths

- README: every PR needs a tracking issue, not only non-trivial work,
  now that the PR description check fails without one.
- AGENTS.md: list the PR description job alongside the rest of PR CI.
- Test that gh api's HTTP 410 (a deleted issue) reports a missing issue,
  like HTTP 404.
- Test that the dependency-update script replaces a "Fixes #" line that
  is the open update PR's whole description.

The new tests cover existing behaviour, so they passed when added.

Part of #861.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: anth-volk

AGENTS.md

@@ -79,6 +79,8 @@ same-repository draft PR.
 - API integration tests live in `projects/policyengine-apis-integ`.
 - PR CI runs simulation unit tests, Ruff format checks, quality guards
   (including the rules registry), Docker build, and local integration tests.
+  A separate `PR description` job checks that every open PR's description
+  starts with `Fixes #ISSUE_NUMBER` for an open issue in this repository.
 - There is currently no repository-wide changelog fragment requirement.
 
 ## Safety Boundaries

README.md

@@ -147,8 +147,9 @@ See [AGENTS.md](AGENTS.md) and
 for the canonical same-repository draft PR workflow.
 
 1. Create a feature branch (never commit to `main`).
-2. Open a GitHub issue for non-trivial work; put `Fixes #ISSUE_NUMBER` as the
-   first line of the PR description.
+2. Open a GitHub issue for the work; put `Fixes #ISSUE_NUMBER` as the first
+   line of the PR description. Every PR needs one: the `PR description` check
+   fails otherwise.
 3. Make changes and run the relevant tests locally.
 4. Open a same-repository draft PR: `make push-pr-branch`, then
    `gh pr create --draft`.

projects/policyengine-simulation-entry/tests/test_pr_description_check.py

@@ -309,13 +309,17 @@ def test_a_pull_request_number_fails(fakes, tmp_path):
     assert "#857 is a pull request, not an issue." in _error_annotation(result)
 
 
-def test_a_missing_issue_fails(fakes, tmp_path):
+@pytest.mark.parametrize(
+    "gh_error",
+    [NOT_FOUND_STDERR, "gh: This issue was deleted (HTTP 410)"],
+)
+def test_a_missing_issue_fails(fakes, tmp_path, gh_error):
     result = _run_check(
         fakes,
         tmp_path,
         "Fixes #999999\n",
         FAKE_GH_STDOUT=NOT_FOUND_STDOUT,
-        FAKE_GH_STDERR=NOT_FOUND_STDERR,
+        FAKE_GH_STDERR=gh_error,
         FAKE_GH_EXIT="1",
     )
 

projects/policyengine-simulation-entry/tests/test_workflow_scripts.py

@@ -461,6 +461,19 @@ def test_create_pr_replaces_an_earlier_fixes_line_on_the_open_update_pr(
     assert descriptions == f"Fixes #870\n{rest}\n"
 
 
+def test_create_pr_replaces_a_fixes_line_that_is_the_whole_description(fakes, tmp_path):
+    result, descriptions = _create_pr(
+        fakes,
+        tmp_path,
+        FAKE_OPEN_PR="900",
+        FAKE_OPEN_PR_BODY="Fixes #860",
+        FAKE_OPEN_ISSUES="870\n",
+    )
+
+    assert result.returncode == 0, result.stderr
+    assert descriptions == "Fixes #870\n"
+
+
 def test_create_pr_leaves_an_open_update_pr_that_names_the_tracking_issue(
     fakes, tmp_path
 ):