feat(cli): allow decnet ttp on agents (DEBT-047)

The TTP-tagging worker is now safe to run on agent hosts: EmailLifter
disk-reaches body-aware predicates from the local artifacts tree
(DEBT-035 unblocked filesystem access; DEBT-047 added the helper).

Drop `ttp` from MASTER_ONLY_COMMANDS in cli/gating.py and remove the
defence-in-depth `_require_master_mode("ttp")` call in cli/ttp.py.
`ttp-backfill` walks the master DB and stays master-only.

tests/cli/test_gating_ttp_agent.py

@@ -0,0 +1,65 @@
+"""Agent-mode gating for the `ttp` worker (DEBT-047).
+
+`decnet ttp` runs the live TTP-tagging worker against local bus events
+and the local artifacts tree. After DEBT-047 it MUST be available on
+agent hosts so EmailLifter R0047 (BEC) can disk-reach .eml files
+without round-tripping raw body text through the master.
+
+`decnet ttp-backfill` walks the master DB and stays master-only.
+"""
+from __future__ import annotations
+
+import os
+import pathlib
+import subprocess
+import sys
+from pathlib import Path
+
+
+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.update(overrides)
+    base["DECNET_CONFIG"] = "/nonexistent/decnet.ini"
+    base.setdefault("DECNET_JWT_SECRET", "x" * 32)
+    return base
+
+
+def _help_text(env: dict[str, str]) -> str:
+    result = subprocess.run(
+        [str(DECNET_BIN), "--help"],
+        env=env, cwd=str(REPO),
+        capture_output=True, text=True, timeout=20,
+    )
+    assert result.returncode == 0, result.stderr
+    return result.stdout
+
+
+def test_ttp_visible_on_agent_mode():
+    out = _help_text(_clean_env(DECNET_MODE="agent", DECNET_DISALLOW_MASTER="true"))
+    assert "ttp " in out or "ttp\n" in out, (
+        "`ttp` worker must be agent-runnable after DEBT-047 (disk-reach unblock)"
+    )
+
+
+def test_ttp_backfill_hidden_on_agent_mode():
+    out = _help_text(_clean_env(DECNET_MODE="agent", DECNET_DISALLOW_MASTER="true"))
+    assert "ttp-backfill" not in out, (
+        "`ttp-backfill` walks the master DB and must stay master-only"
+    )
+
+
+def test_ttp_visible_on_master_mode():
+    out = _help_text(_clean_env(DECNET_MODE="master"))
+    assert "ttp " in out or "ttp\n" in out
+    assert "ttp-backfill" in out
+
+
+def test_master_only_set_excludes_ttp():
+    """Source-level guard against re-adding `ttp` to the master-only set."""
+    from decnet.cli.gating import MASTER_ONLY_COMMANDS
+    assert "ttp" not in MASTER_ONLY_COMMANDS
+    assert "ttp-backfill" in MASTER_ONLY_COMMANDS