feat(fleet): opus-fable adoption — doctrine + discipline hooks (v0.18.0 A)

Fleet behaves more like Fable 5 on existing model tiers, behind
ROBOCO_FABLE_MODE_ENABLED (config default off; armed :-true on the NAS compose,
absent from the registry compose).

- Doctrine: vendored agents/prompts/doctrine/fable.md composed into every
  agent's system prompt via fable_doctrine_layer() after base.md.
- Hooks (Claude Code): 4 non-overlapping hooks (stop-gate/bash-discipline/
  honesty-nudge/precompact) appended per-agent via _fable_hook_groups(). The
  make-quality + lint-suppression duplicates are deliberately NOT added (already
  gate-enforced); session-start skipped.
- Hooks (grok): conservative V1 — only the non-denying honesty-nudge, since a
  grok hook deny cancels the whole run.
- Flag on the feature-flags card; hook scripts shipped into the agent image.

Flag-off spawn path proven byte-identical (worktree diff, sha256 match); full
suite green (2074 unit + e2e-smoke + hook harness), mypy/xenon/ruff clean.
Fixed a real stdin bug in the vendored stop-gate hook (heredoc + pipe both
claimed stdin). Distilled from rennf93/opus-fable-playbook (MIT).

tests/e2e_smoke/test_dev_lifecycle.py

@@ -10,6 +10,7 @@ envelope prints verbatim so a seam regression names itself.
 from __future__ import annotations
 
 from typing import TYPE_CHECKING
+from unittest.mock import patch
 
 from tests.e2e_smoke.arcs import (
     dev_arc,
@@ -55,3 +56,44 @@ def test_leaf_dev_task_reaches_pm_review(e2e_stack: E2EStack) -> None:
     final = task_state(stack, task_id)
     assert final["status"] == "awaiting_pm_review", final
     assert final["docs_complete"] is True, final
+
+
+def test_leaf_dev_task_reaches_pm_review_with_fable_mode_on(
+    e2e_stack: E2EStack,
+) -> None:
+    """Non-interference regression check for fable_mode_enabled=True.
+
+    This harness cannot exercise compose_prompt / _generate_agent_settings —
+    those live entirely in the orchestrator's spawn-prep path, which the
+    harness bypasses by design (see tests/integration/test_fable_mode_spawn_prep.py
+    for that half). What it CAN prove is that arming the flag doesn't perturb
+    the gateway/lifecycle arc itself: the same scenario must reach the same
+    outcome with the flag on as with it off.
+    """
+    with patch("roboco.config.settings.fable_mode_enabled", True):
+        stack = e2e_stack
+        company = seed_company(stack)
+        project_id, project_slug = seed_project(stack, company)
+        task_id = seed_task(
+            stack,
+            title="Add the greeting module (fable-mode non-interference check)",
+            description=(
+                "Create greeting.txt with a friendly greeting so the smoke "
+                "harness has a real file change to commit, push, and merge."
+            ),
+            acceptance_criteria=[
+                "greeting.txt exists at the repo root",
+                "its content greets the reader",
+            ],
+            project_id=project_id,
+            created_by=company.cell_pm_id,
+            assigned_to=company.dev_id,
+        )
+
+        dev_arc(stack, company, project_slug, task_id)
+        qa_arc(stack, company, task_id)
+        doc_arc(stack, company, task_id, filename="greeting.txt")
+
+        final = task_state(stack, task_id)
+    assert final["status"] == "awaiting_pm_review", final
+    assert final["docs_complete"] is True, final