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/integration/test_fable_mode_spawn_prep.py

@@ -0,0 +1,61 @@
+"""Fable-mode ships doctrine + hooks together at a single agent spawn.
+
+Task 3's and Task 6's unit tests each prove their own half in isolation:
+compose_prompt includes the doctrine layer; _generate_agent_settings injects
+the hook groups. Neither proves the two halves land together for the SAME
+spawn. This is that proof, using the orchestrator's own
+_generate_composed_prompt + _generate_agent_settings directly — the plan's
+named lighter-weight alternative to _prepare_agent_spawn, which needs a live
+DB session, docker, and a real workspace/worktree and so is not a fit for a
+fast integration test (see the plan doc under docs/superpowers/plans/,
+Task 11).
+"""
+
+from __future__ import annotations
+
+import json
+from unittest.mock import patch
+
+from roboco.runtime.orchestrator import AgentOrchestrator
+
+_WS = "/data/workspaces/roboco-api/backend/be-dev-1"
+_CELL = "/data/workspaces/roboco-api/backend"
+
+
+def _orch() -> AgentOrchestrator:
+    with patch.object(AgentOrchestrator, "__init__", return_value=None):
+        return AgentOrchestrator.__new__(AgentOrchestrator)
+
+
+def test_flag_on_ships_doctrine_and_hooks_together() -> None:
+    with patch("roboco.config.settings.fable_mode_enabled", True):
+        orch = _orch()
+        prompt_path = orch._generate_composed_prompt("be-dev-1")
+        settings_path = orch._generate_agent_settings(
+            agent_id="be-dev-1",
+            role="developer",
+            workspace_path=_WS,
+            cell_workspace_path=_CELL,
+        )
+    prompt = prompt_path.read_text()
+    hooks = json.loads(settings_path.read_text())["hooks"]
+    assert "# Fable Doctrine" in prompt
+    stop_cmds = [h["command"] for g in hooks["Stop"] for h in g["hooks"]]
+    assert "/app/scripts/fable-stop-gate-hook.sh" in stop_cmds
+
+
+def test_flag_off_ships_neither() -> None:
+    with patch("roboco.config.settings.fable_mode_enabled", False):
+        orch = _orch()
+        prompt_path = orch._generate_composed_prompt("be-dev-1")
+        settings_path = orch._generate_agent_settings(
+            agent_id="be-dev-1",
+            role="developer",
+            workspace_path=_WS,
+            cell_workspace_path=_CELL,
+        )
+    prompt = prompt_path.read_text()
+    hooks = json.loads(settings_path.read_text())["hooks"]
+    assert "# Fable Doctrine" not in prompt
+    stop_cmds = [h["command"] for g in hooks["Stop"] for h in g["hooks"]]
+    assert not any("fable" in c for c in stop_cmds)