The queue: parking, the operator's order, blocked items (M2.2)

Several work items on the new loop (L1, D4):
- Switching parks the item on the workspace: its work is committed and its
  branch pushed; resuming checks the item's branch back out. Branches of
  open items are never set aside by a fresh workspace.
- Next item: the operator's "do #N next" first, otherwise the oldest.
- Issues with open Gitea dependencies are blocked until they close, checked
  at intake and on every round; status shows blocked items and the line.

Tests: queue unit tests, the property walk over several issues with
dependencies and a new parked_work_safe property (it catches parking
disabled), and an end-to-end test with two issues where B depends on A.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPQ14L3LikyefCMFBqEbKE

nisse-dev/tests/test_loop_milestones.py

@@ -58,11 +58,19 @@ class Forge:
         self.notes: dict[int, list[ReviewNote]] = {}
         self.posted: list[tuple[int, str]] = []
         self.closed_issues: list[int] = []
-        self.issue = Issue(14, "Add count_words", "count words", "don", ())
+        self.issues = [Issue(14, "Add count_words", "count words", "don", ())]
+        self.deps: dict[int, list[int]] = {}  # issue -> the issues it depends on
 
     # intake
     def assigned_issues(self, repo: str, assignee: str) -> list[Issue]:
-        return [] if self.issue.number in self.closed_issues else [self.issue]
+        return [i for i in self.issues if i.number not in self.closed_issues]
+
+    def open_dependencies(self, repo: str, index: int) -> list[str]:
+        return [
+            f"{repo}#{n}"
+            for n in self.deps.get(index, [])
+            if n not in self.closed_issues
+        ]
 
     def assigned_by(self, repo: str, index: int, assignee: str) -> str:
         return "don"
@@ -109,14 +117,31 @@ class Forge:
 
     # what the operator does in Gitea
     def merge(self, n: int) -> None:
-        head = self.prs[n]["head"]
-        git(
-            "--git-dir",
-            str(self.bare),
-            "update-ref",
-            "refs/heads/main",
-            f"refs/heads/{head}",
-        )
+        head = f"refs/heads/{self.prs[n]['head']}"
+        bare = ["--git-dir", str(self.bare)]
+        merged = head
+        if subprocess.run(
+            ["git", *bare, "merge-base", "--is-ancestor", "refs/heads/main", head],
+            check=False,
+        ).returncode:
+            # Main moved on (another item's PR): a real merge commit.
+            tree = git(*bare, "merge-tree", "--write-tree", "refs/heads/main", head)
+            merged = git(
+                *bare,
+                "-c",
+                "user.name=g",
+                "-c",
+                "user.email=g@t",
+                "commit-tree",
+                tree,
+                "-p",
+                "refs/heads/main",
+                "-p",
+                head,
+                "-m",
+                f"Merge PR {n}",
+            )
+        git(*bare, "update-ref", "refs/heads/main", merged)
         self.prs[n].update(state="closed", merged=True)
 
     def files(self, n: int) -> list[str]: