Publish work the executor committed itself, and gate when there is none

On the first live run the executor committed its milestone itself, as
its plan asked. The publisher only published a commit it made, found a
clean tree, reported "no changes to publish", and left the agent in
EXECUTING, where every message is answered "busy".

- publish(): with nothing left to commit, publish anyway when the branch
  has commits origin/<base> lacks; report no changes only when it has
  none.
- push_followup(): same for review fixes, comparing HEAD with what the
  remote branch holds (ls-remote; pushes go by URL, so there is no
  tracking ref).
- Nothing to publish now gates to AWAIT_INPUT, and the reply returns to
  the still-approved milestone plan.

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

nisse-dev/tests/test_pr_open.py

@@ -10,7 +10,7 @@ import pytest
 from nisse_dev.canonical import AssistantText, CanonicalEvent, StoredEvent, ToolCall
 from nisse_dev.control import (
     AWAIT_INPUT,
-    EXECUTING,
+    AWAIT_MILESTONE_SIGNOFF,
     IDLE,
     PR_OPEN,
     Orchestrator,
@@ -153,11 +153,17 @@ def test_unknown_command_in_pr_open_explains_the_options(tmp_path: Path) -> None
     assert "next milestone" in transport.outbound[-1].text
 
 
-def test_no_changes_reports_without_entering_pr_open(tmp_path: Path) -> None:
+def test_no_changes_gates_to_the_operator_without_entering_pr_open(
+    tmp_path: Path,
+) -> None:
     publisher = FakePublisher(result=PublishResult(None, "no changes to publish"))
     store, transport = run_to_pr(tmp_path, publisher)
-    assert store.get_state().state == EXECUTING
-    assert transport.outbound[-1].text == "no changes to publish"
+    # Not left in EXECUTING, where every message is answered "busy".
+    assert store.get_state().state == AWAIT_INPUT
+    assert "no changes to publish" in transport.outbound[-1].text
+    transport.receive("don", "ok")
+    assert store.get_state().state == AWAIT_MILESTONE_SIGNOFF
+    assert "start milestone 1" in transport.outbound[-1].text
 
 
 def test_publish_failure_gates_to_the_operator(tmp_path: Path) -> None: