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

nisse-dev/src/nisse_dev/git.py

@@ -210,8 +210,10 @@ class GitRepo:
         self._git("config", "user.name", name)
         self._git("config", "user.email", email)
 
-    def checkout_new_branch(self, name: str) -> None:
-        self._git("checkout", "-q", "-b", name)
+    def checkout_new_branch(self, name: str, start: str | None = None) -> None:
+        """A new branch, at ``start`` if given. Uncommitted changes come
+        along; git refuses if one would conflict with ``start``."""
+        self._git("checkout", "-q", "-b", name, *([start] if start else []))
 
     def checkout_existing_branch(self, name: str) -> None:
         self._git("checkout", "-q", name)
@@ -300,6 +302,46 @@ class GitRepo:
             "push", "-q", remote_url, f"HEAD:refs/heads/{branch}", credential=credential
         )
 
+    def push_commit(
+        self,
+        remote_url: str,
+        sha: str,
+        branch: str,
+        *,
+        credential: PushCredential | None = None,
+    ) -> None:
+        """Point the remote's ``branch`` at ``sha``, whatever it was."""
+        self._git(
+            "push",
+            "-q",
+            "-f",
+            remote_url,
+            f"{sha}:refs/heads/{branch}",
+            credential=credential,
+        )
+
+    def stash_all(self, message: str) -> str | None:
+        """Take every uncommitted change, untracked files included and build
+        artifacts not, off the work tree as a stash commit; returns its sha,
+        or None when there was nothing. The commit is kept by pushing it."""
+        if not self.has_changes():
+            return None
+        self._git("stash", "push", "-q", "-u", "-m", message)
+        sha = self._git("rev-parse", "stash@{0}")
+        self._git("stash", "drop", "-q")
+        return sha
+
+    def apply_stash(self, sha: str) -> None:
+        """Put a stash commit's changes back on the work tree, uncommitted."""
+        self._git("stash", "apply", "-q", sha)
+
+    def has_commit(self, sha: str) -> bool:
+        try:
+            self._git("cat-file", "-e", f"{sha}^{{commit}}")
+        except GitError:
+            return False
+        return True
+
     def discard_branch(self, branch: str, *, back_to: str) -> None:
         """Throw away a branch and everything on it.