feat(emailgen): gate as master-only

Two-layer gating per CLAUDE.md:
- registration-time: emailgen added to MASTER_ONLY_GROUPS so agents
  don't see the sub-app in 'decnet --help' at all.
- body-guard: _require_master_mode('emailgen ...') at the top of every
  sub-command body so a direct callable import (third-party tooling)
  still bails on agent hosts.

Matches the convention used for 'swarm', 'topology', 'geoip'.  SWARM
agents push their generated mail through the master's emailgen worker
(or none at all); cross-agent emailgen federation stays out of scope.

tests/cli/test_emailgen_gating.py

@@ -0,0 +1,111 @@
+"""``decnet emailgen`` is master-only.
+
+Two layers per CLAUDE.md:
+
+* registration-time hide via :data:`MASTER_ONLY_GROUPS` so agents don't
+  see ``decnet emailgen`` in ``--help`` at all,
+* body-guard ``_require_master_mode()`` so a direct callable import (e.g.
+  from a third-party tool) still bails on agent hosts.
+"""
+from __future__ import annotations
+
+import json
+import os
+import pathlib
+import subprocess
+import sys
+from pathlib import Path
+
+import pytest
+from typer.testing import CliRunner
+
+
+REPO = pathlib.Path(__file__).resolve().parent.parent.parent
+DECNET_BIN = Path(sys.executable).parent / "decnet"
+
+
+def _clean_env(**overrides: str) -> dict[str, str]:
+    base = {"PATH": os.environ["PATH"], "HOME": "/nonexistent-for-test"}
+    base["DECNET_CONFIG"] = "/nonexistent/decnet.ini"
+    base.setdefault("DECNET_JWT_SECRET", "x" * 32)
+    base.update(overrides)
+    return base
+
+
+def test_emailgen_visible_in_master_mode():
+    result = subprocess.run(
+        [str(DECNET_BIN), "--help"],
+        env=_clean_env(DECNET_MODE="master"),
+        cwd=str(REPO),
+        capture_output=True, text=True, timeout=20,
+    )
+    assert result.returncode == 0
+    assert "emailgen" in result.stdout
+
+
+def test_emailgen_hidden_in_agent_mode():
+    result = subprocess.run(
+        [str(DECNET_BIN), "--help"],
+        env=_clean_env(DECNET_MODE="agent", DECNET_DISALLOW_MASTER="true"),
+        cwd=str(REPO),
+        capture_output=True, text=True, timeout=20,
+    )
+    assert result.returncode == 0
+    # The sub-app's help string must be gone too — bare "emailgen" can
+    # appear in other command descriptions.
+    assert "Drip persona-driven fake corporate email" not in result.stdout
+
+
+def test_emailgen_subprocess_run_rejects_in_agent_mode():
+    """Subprocess-level: a fresh Python invocation of `decnet emailgen
+    run` under DECNET_MODE=agent must exit non-zero (gate hides the
+    sub-app, so the command is unknown to Typer)."""
+    result = subprocess.run(
+        [str(DECNET_BIN), "emailgen", "run"],
+        env=_clean_env(DECNET_MODE="agent", DECNET_DISALLOW_MASTER="true"),
+        cwd=str(REPO),
+        capture_output=True, text=True, timeout=20,
+    )
+    assert result.returncode != 0
+
+
+def test_emailgen_subprocess_import_personas_rejects_in_agent_mode(tmp_path):
+    src = tmp_path / "personas.json"
+    src.write_text(json.dumps([{
+        "name": "X", "email": "x@y.com", "role": "X", "tone": "formal",
+        "mannerisms": [],
+    }, {
+        "name": "Y", "email": "y@y.com", "role": "Y", "tone": "formal",
+        "mannerisms": [],
+    }]))
+    result = subprocess.run(
+        [str(DECNET_BIN), "emailgen", "import-personas", str(src)],
+        env=_clean_env(DECNET_MODE="agent", DECNET_DISALLOW_MASTER="true"),
+        cwd=str(REPO),
+        capture_output=True, text=True, timeout=20,
+    )
+    assert result.returncode != 0
+
+
+def test_require_master_mode_body_guard_fires_directly(monkeypatch):
+    """Defence-in-depth: even bypassing Typer registration, the body-level
+    ``_require_master_mode('emailgen ...')`` raises ``typer.Exit``.  Same
+    mechanism is verified for `api`/`deploy` in test_mode_gating.py."""
+    import typer
+
+    from decnet.cli.gating import _require_master_mode
+
+    monkeypatch.setenv("DECNET_MODE", "agent")
+    monkeypatch.setenv("DECNET_DISALLOW_MASTER", "true")
+
+    with pytest.raises(typer.Exit):
+        _require_master_mode("emailgen run")
+
+
+def test_master_mode_falls_through_body_guard(monkeypatch):
+    """In master mode the guard is a no-op (raises nothing)."""
+    from decnet.cli.gating import _require_master_mode
+
+    monkeypatch.setenv("DECNET_MODE", "master")
+    # Should simply return.
+    _require_master_mode("emailgen run")