refactor: prepare the interactive shell in its own module

The prompt-hook step (#41) pushed linux.py past its line count in the
quality baseline. The three steps that make netmiko's fresh shell fit to
read - history off, prompt hooks off, no line wrap - and the prompt
pattern move to napalm_linux/_shell.py, under strict mypy. linux.py
shrinks below where it was; the baseline follows.

tests/test_systemd_osc_context.py

@@ -11,7 +11,8 @@ characters and netOrk could not store the poll.
 
 from unittest.mock import MagicMock, patch
 
-from napalm_linux.linux import _HISTORY_OFF, _PROMPT_HOOKS_OFF, LinuxDriver
+from napalm_linux._shell import HISTORY_OFF, PROMPT_HOOKS_OFF
+from napalm_linux.linux import LinuxDriver
 
 #: What ``cat /etc/os-release | grep PRETTY_NAME ...`` returned through netmiko on
 #: an Ubuntu 26.04.1 host (2026-10-23), the prompt line already gone.
@@ -39,13 +40,13 @@ class TestPromptHooksOff:
         _opened(device)
 
         sent = [c.args[0] for c in device.send_command.call_args_list]
-        assert sent[:2] == [_HISTORY_OFF, _PROMPT_HOOKS_OFF]
+        assert sent[:2] == [HISTORY_OFF, PROMPT_HOOKS_OFF]
 
     def test_it_unsets_both_hooks_including_an_array_prompt_command(self):
         # The script appends to PROMPT_COMMAND as an array; assigning the empty
         # string would only clear element 0 and leave systemd's hook running.
-        assert _PROMPT_HOOKS_OFF.startswith(" ")
-        assert "unset PS0 PROMPT_COMMAND" in _PROMPT_HOOKS_OFF
+        assert PROMPT_HOOKS_OFF.startswith(" ")
+        assert "unset PS0 PROMPT_COMMAND" in PROMPT_HOOKS_OFF
 
     def test_the_prompt_is_read_again_once_the_hooks_are_gone(self):
         # netmiko learned the prompt with the sequences in front of it; a prompt
@@ -56,7 +57,7 @@ class TestPromptHooksOff:
         _opened(device)
 
         calls = [(c[0], c.args[0] if c.args else None) for c in device.mock_calls]
-        hooks_off = calls.index(("send_command", _PROMPT_HOOKS_OFF))
+        hooks_off = calls.index(("send_command", PROMPT_HOOKS_OFF))
         assert "set_base_prompt" in [name for name, _ in calls[hooks_off + 1 :]]
 
     def test_a_shell_that_refuses_it_does_not_stop_the_connection(self):