Test levels: smoke e2e on PRs, the full set on e2e:full and before a release

- Every PR: lint, unit tests across the runner's cores (pytest-xdist), 16
  property walks from a seed that moves with the run number, integration
  tests, and the end-to-end smoke tests (the slice, a conversation filed
  as work, a question; `smoke` marker).
- A PR labelled e2e:full, a push to main, or a manual run: all 48 walks and
  the whole end-to-end suite. Adding e2e:full to a PR runs it; any other
  label runs nothing.
- A published release runs the full set before its image is built.
- The walk's seeds come from NISSE_WALK_SEEDS / NISSE_WALK_FIRST (48 from 0
  by default).

Measured here: the unit tier on 2 workers took 3m49s against 6-8 minutes
serially.

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

@@ -239,6 +239,7 @@ class Operator:
         return self.transport.outbound[-1].text if self.transport.outbound else ""
 
 
+@pytest.mark.smoke
 def test_the_slice_end_to_end(
     running: tuple[RunningLoop, InMemoryTransport, str],
 ) -> None:
@@ -501,6 +502,7 @@ def test_a_two_milestone_feature(
         operator.tidy(issues, prs)
 
 
+@pytest.mark.smoke
 def test_a_conversation_becomes_filed_work(
     running: tuple[RunningLoop, InMemoryTransport, str],
 ) -> None:
@@ -636,6 +638,7 @@ def test_a_conversation_becomes_a_new_project(
         assert any(f.startswith("docs/plans/") for f in files), files
 
 
+@pytest.mark.smoke
 def test_a_question_leaves_the_restatement_up(
     running: tuple[RunningLoop, InMemoryTransport, str],
 ) -> None: