feat(gateway): WSL-aware gateway with smart systemd detection (#7510)

- Add shared is_wsl() to hermes_constants (like is_termux)
- Update supports_systemd_services() to verify systemd is actually
  running on WSL before returning True
- Add WSL-specific guidance in gateway install/start/setup/status
  for both cases: WSL+systemd and WSL without systemd
- Improve help strings: 'run' now says recommended for WSL/Docker,
  'start'/'install' now mention systemd/launchd explicitly
- Add WSL gateway FAQ section with tmux/nohup/Task Scheduler tips
- Update CLI commands docs with WSL tip
- Deduplicate _is_wsl() from clipboard.py to shared hermes_constants
- Fix clipboard tests to reset hermes_constants cache
- 20 new WSL-specific tests covering detection, systemd check,
  supports_systemd_services integration, and command output

Motivated by user feedback: took 1 hour to figure out run vs start
on WSL, Telegram bot kept disconnecting due to flaky WSL systemd.

tests/tools/test_clipboard.py

@@ -205,9 +205,9 @@ class TestMacosOsascript:
 
 class TestIsWsl:
     def setup_method(self):
-        # Reset cached value before each test
-        import hermes_cli.clipboard as cb
-        cb._wsl_detected = None
+        # _is_wsl is now hermes_constants.is_wsl — reset its cache
+        import hermes_constants
+        hermes_constants._wsl_detected = None
 
     def test_wsl2_detected(self):
         content = "Linux version 5.15.0 (microsoft-standard-WSL2)"
@@ -229,6 +229,7 @@ class TestIsWsl:
             assert _is_wsl() is False
 
     def test_result_is_cached(self):
+        import hermes_constants
         content = "Linux version 5.15.0 (microsoft-standard-WSL2)"
         with patch("builtins.open", mock_open(read_data=content)) as m:
             assert _is_wsl() is True