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

pyproject.toml

@@ -4,6 +4,7 @@ members = ["nisse-core", "nisse-dev", "nisse-ops"]
 [dependency-groups]
 dev = [
     "pytest>=8",
+    "pytest-xdist>=3",
     "ruff>=0.6",
 ]
 
@@ -19,6 +20,7 @@ addopts = "--strict-markers --ignore=tests/integration/seed"
 markers = [
     "integration: end-to-end tests that exercise installed entry points or external services",
     "e2e: the whole agent against real services, run after each milestone (scripts/e2e.sh)",
+    "smoke: the end-to-end tests every PR runs; the rest on e2e:full and before a release",
 ]
 
 [tool.ruff]