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).
