feat: add --tmux/-t option to run agents in attachable tmux sessions

Wraps the backend in 'tmux new-session -A' with a per-backend, per-directory
session name, so agent sessions survive SSH drops and re-running the same
command reattaches. Falls back to a direct run when already inside tmux,
and errors clearly when tmux is not installed.

tests/test_entrypoints.py

@@ -143,3 +143,80 @@ class TestMyhermesEntrypoint:
                 "abc123",
                 "--no-restore-cwd",
             ]
+
+
+class TestTmuxOption:
+    """``--tmux`` / ``-t`` wraps the backend in an attachable tmux session."""
+
+    def _which(self, name: str) -> str:
+        return f"/usr/bin/{name}"
+
+    def test_tmux_wraps_command(self, tmp_path: Path) -> None:
+        runner = CliRunner()
+        with (
+            patch("myagents.launcher.shutil.which", side_effect=self._which),
+            patch("myagents.launcher.subprocess.run") as mock_run,
+            patch.dict("os.environ", {}, clear=False) as env,
+        ):
+            env.pop("TMUX", None)
+            mock_run.return_value = MagicMock(returncode=0)
+            result = runner.invoke(claude_cli, ["--cwd", str(tmp_path), "--tmux"])
+            assert result.exit_code == 0
+            argv = mock_run.call_args[0][0]
+            assert argv[:3] == ["/usr/bin/tmux", "new-session", "-A"]
+            assert "-c" in argv and str(tmp_path) in argv
+            assert argv[-1] == "/usr/bin/claude --dangerously-skip-permissions"
+
+    def test_tmux_session_name_deterministic(self, tmp_path: Path) -> None:
+        from myagents.launcher import _tmux_session_name
+
+        first = _tmux_session_name("claude", tmp_path)
+        second = _tmux_session_name("claude", tmp_path)
+        other = _tmux_session_name("kimi", tmp_path)
+        assert first == second
+        assert first != other
+        assert first.startswith("myclaude-")
+
+    def test_tmux_with_passthrough(self, tmp_path: Path) -> None:
+        runner = CliRunner()
+        with (
+            patch("myagents.launcher.shutil.which", side_effect=self._which),
+            patch("myagents.launcher.subprocess.run") as mock_run,
+            patch.dict("os.environ", {}, clear=False) as env,
+        ):
+            env.pop("TMUX", None)
+            mock_run.return_value = MagicMock(returncode=0)
+            result = runner.invoke(
+                claude_cli, ["--cwd", str(tmp_path), "-t", "-r", "sess-1"]
+            )
+            assert result.exit_code == 0
+            argv = mock_run.call_args[0][0]
+            assert argv[0] == "/usr/bin/tmux"
+            assert argv[-1].endswith("-r sess-1")
+
+    def test_tmux_skipped_when_already_inside(self, tmp_path: Path) -> None:
+        runner = CliRunner()
+        with (
+            patch("myagents.launcher.shutil.which", side_effect=self._which),
+            patch("myagents.launcher.subprocess.run") as mock_run,
+            patch.dict("os.environ", {"TMUX": "/tmp/tmux-501/default,1,0"}),
+        ):
+            mock_run.return_value = MagicMock(returncode=0)
+            result = runner.invoke(claude_cli, ["--cwd", str(tmp_path), "-t"])
+            assert result.exit_code == 0
+            argv = mock_run.call_args[0][0]
+            assert argv[0] == "/usr/bin/claude"
+
+    def test_tmux_missing_binary(self, tmp_path: Path) -> None:
+        runner = CliRunner()
+
+        def _which(name: str) -> str | None:
+            return None if name == "tmux" else f"/usr/bin/{name}"
+
+        with (
+            patch("myagents.launcher.shutil.which", side_effect=_which),
+            patch.dict("os.environ", {}, clear=False) as env,
+        ):
+            env.pop("TMUX", None)
+            result = runner.invoke(claude_cli, ["--cwd", str(tmp_path), "-t"])
+            assert result.exit_code == 127