refactor(cli): extract 32 slash-command handlers into CLICommandsMixin (god-file Phase 4)

Lift the `_handle_*_command` cluster (2,077 LOC) out of HermesCLI into
hermes_cli/cli_commands_mixin.py; HermesCLI now inherits CLICommandsMixin so
every self.<handler> call resolves unchanged via the MRO. Behavior-neutral.

Import discipline mirrors gateway/slash_commands.py (PR #41886): neutral deps
imported at the mixin module top level; cli.py-internal helpers/constants
(_cprint, _ACCENT, save_config_value, ...) imported lazily inside each handler
via 'from cli import ...' so the mixin never imports cli at module scope.

cli.py 16215 -> 14139 LOC. One test mock repointed (cli.is_browser_debug_ready
-> hermes_cli.cli_commands_mixin.is_browser_debug_ready).

tests/cli/test_cli_browser_connect.py

@@ -238,7 +238,7 @@ class TestChromeDebugLaunch:
         cli._pending_input = Queue()
         monkeypatch.delenv("BROWSER_CDP_URL", raising=False)
 
-        with patch("cli.is_browser_debug_ready", return_value=True), \
+        with patch("hermes_cli.cli_commands_mixin.is_browser_debug_ready", return_value=True), \
              patch("tools.browser_tool.cleanup_all_browsers"), \
              patch("tools.browser_tool._ensure_cdp_supervisor"), \
              redirect_stdout(StringIO()):