Bootstrap self-improvement loop, decompose model-divergence-enforcement

Self-improvement loop:
- Created via --create-loop --from-template self-improvement
- Scheduled via launchd (3600s interval)
- State: running

model-divergence-enforcement task:
- Research approved, decomposed into 3 sequential subtasks:
  1. mde-manifest-detection (models.json + detect_models.py)
  2. mde-interactive-enforcement (conflict matrix + --model args + audit)
  3. mde-loop-enforcement (loop.json roles + {model} substitution + check-gate)
- Parent at decomposition:approved (stays active until subtasks complete)
- Each subtask has BRIEF.md with scope, deliverables, acceptance criteria

tasks/model-divergence-enforcement/subtasks/mde-loop-enforcement/BRIEF.md

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+# BRIEF — mde-loop-enforcement
+
+**Parent:** model-divergence-enforcement
+**Subtask:** 3 of 3 (depends on mde-interactive-enforcement)
+**Spec ref:** `tasks/model-divergence-enforcement/SPEC.md` §Subtask 3
+**Design ref:** `design/framework/technical.md` §§10, §13
+
+## Objective
+
+Add loop-level model-divergence enforcement: `loop.json` per-role `model` field,
+`{model}` substitution in `_invoke_harness`, and `--check-gate` model-divergence
+brake gate.
+
+## Deliverables
+
+1. `scripts/loop-runner.py`:
+   - `_invoke_harness` (line 366-404): `{model}` placeholder substitution from `loop.json` role config
+2. `scripts/status.py`:
+   - `--check-gate`: model-divergence brake gate (loop-verify ≠ loop-implement in multi-LLM mode)
+   - `cmd_create_loop` / `cmd_install_schedule`: validate `loop.json` per-role `model` fields
+3. `templates/loops/`: update loop templates with `roles` schema example
+4. `design/loops/technical.md`: document `{model}` substitution
+5. `tests/test_model_divergence.py`: loop model binding, `{model}` substitution, check-gate halt
+
+## Acceptance
+
+- `loop.json` with `roles.implementer.model` → `_invoke_harness` substitutes `{model}` in harness command
+- `loop.json` without per-role `model` → defaults to `models.json` `default` model
+- `--check-gate` in multi-LLM mode halts if loop-verify model = loop-implement model
+- `--check-gate` in single-LLM mode does NOT halt (advisory only)
+- `pytest tests/ -q` green