Multi-milestone work and documents on the new loop (M2.4)

- Every milestone branch is cut from the latest default branch: the next
  milestone builds on the merged previous one, and a redo after a closed PR
  drops the rejected work. Uncommitted work comes along.
- While the item's PR is open, start_milestone is refused, with the PR's
  state read from Gitea at that moment, not the last watch round.
- finish_milestone(final): earlier PRs say "Part of #N", the last "Closes
  #N"; unsaid, it's final, as before.
- close_item with the item's PR still open waits for the merge (L2: a
  closed item leaves nothing open).
- The driver's instructions cover ARCHITECTURE.md and build plans under
  docs/plans/ (D2); the harness never parses a plan.
- Parking stashes uncommitted work and pushes it to <item>-parked, put back
  on resume, instead of committing it onto the item's branch. The walk
  found a half-done review fix parked onto a PR that had merged meanwhile,
  where it was lost. Architecture D4's parking line says so.

Tests: unit tests for fresh milestone branches, the merge wait (including
a merge not yet watched), redo after a closed PR, Part of/Closes, close
waiting on an open PR, and the parked-fix regression; the walk's new
milestones_start_fresh property (fork-from-current fails 9 seeds); an e2e
two-milestone feature.

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

tests/integration/test_end_to_end_loop.py

@@ -425,3 +425,59 @@ def test_a_plain_yes_is_a_go_ahead(
             prs.append(pr)
     finally:
         operator.tidy(issues, prs)
+
+
+def test_a_two_milestone_feature(
+    running: tuple[RunningLoop, InMemoryTransport, str],
+) -> None:
+    """M2.4: work in two milestones. The first PR carries the build plan and
+    is part of the issue; the second starts from the merged first and closes
+    it; the item closes after both merge."""
+    agent, transport, run = running
+    operator = Operator(agent, transport)
+    gitea, repo = agent.gitea, agent.config.gitea_repo
+    as_operator = operator_gitea(agent.config.gitea_url)
+    issues: list[int] = []
+    prs: list[int] = []
+    try:
+        body = (
+            f"{task(run)}\n\nThen, separately, {dependent_task(run)[0].lower()}"
+            f"{dependent_task(run)[1:]}\n\nDo this in two milestones, one PR each: "
+            f"vowels_{run} first, then consonants_{run} after the first has merged. "
+            "Write a short build plan for it."
+        )
+        issues.append(as_operator.create_issue(repo, f"letters_{run}", body))
+        as_operator._request(
+            "PATCH",
+            f"/repos/{repo}/issues/{issues[0]}",
+            json={"assignees": [agent.account]},
+        )
+        operator.tick()
+
+        # --- milestone 1: the plan and vowels; part of the issue ---
+        item = operator.drive_until(MERGE, issues[0])
+        first = loop_of(item).pr
+        prs.append(first)
+        pr = gitea._request("GET", f"/repos/{repo}/pulls/{first}")
+        assert f"Part of #{issues[0]}" in pr["body"], pr["body"]
+        files = [
+            f["filename"]
+            for f in gitea._request("GET", f"/repos/{repo}/pulls/{first}/files")
+        ]
+        assert any(f.startswith("docs/plans/") for f in files), files
+        merge(as_operator, repo, first)
+        operator.tick()
+
+        # --- milestone 2: from the merged first; closes the issue ---
+        item = operator.drive_until(MERGE, issues[0])
+        second = loop_of(item).pr
+        assert second != first
+        prs.append(second)
+        pr = gitea._request("GET", f"/repos/{repo}/pulls/{second}")
+        assert f"Closes #{issues[0]}" in pr["body"], pr["body"]
+        diff = gitea._http.get(f"/repos/{repo}/pulls/{second}.diff").text
+        assert f"consonants_{run}" in diff
+        assert f"\n+def vowels_{run}" not in diff  # built on the merged first
+        operator.merge_and_close(issues[0])
+    finally:
+        operator.tidy(issues, prs)