feat(slots): Phase 3 task 6.3 — model-disabled, slot-contracted deterministic stages

Express the deterministic scan stages as model-disabled processors carrying work in
named slots under a read/write allow-list validated on every lifecycle hook.

- slot_contract.py (zero-dep): SlotContractError, SlotContractMixin (reads_slots/
  writes_slots/skip_model=True, validates reads pre-hook and writes post-hook),
  SlotPipeline (runs stages in order, enforces the model-disabled invariant, emits
  stage_start/stage_end lifecycle hooks). Preserves the ProcessorSpec.reads/writes
  provenance discipline without forcing deterministic stages through HarnessX's agent
  loop ("bridge, do not map").
- stage_processors.py: Preprocess/EntryPoint/Rank/QuickFindings/PolicyScoring/Score as
  skip_model slot processors; run_quick_pipeline is byte-identical to the quick path.
  host_under_harnessx is the lazy, capability-gated bridge that disables the model on
  HarnessX's BeforeModelEvent.skip_model (proven against the real API when the extra
  is present).
- 8.4 zero-dep guard hardened: now hermetic (subprocess) and covers the two new
  modules; the module-level import scan flags only column-0 imports so the intended
  lazy import inside host_under_harnessx is allowed.

The sync HarnessRuntime stays the top-level driver (design "stays through Phase 4");
these slot processors are the model-disabled representation, ready for Phase 5.

tests/test_slot_contract.py (8 tests). 153 passed, 3 skipped; ruff + mypy clean;
detection gate PASS.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

.kiro/specs/harnessx-self-improving-rulesets/spec.json

@@ -1,7 +1,7 @@
 {
   "feature_name": "harnessx-self-improving-rulesets",
   "created_at": "2026-06-22T10:07:18Z",
-  "updated_at": "2026-06-22T16:20:00Z",
+  "updated_at": "2026-06-22T16:45:00Z",
   "language": "en",
   "phase": "implementation",
   "approvals": {