feat(cli,gateway): /new accepts optional session name argument

Allow users to start a fresh session and immediately set its title by
passing a name to /new (or /reset):

    /new Refactor auth module

Changes:
- hermes_cli/commands.py: add args_hint='[name]' to /new command
- cli.py: parse title argument in process_command(), pass to new_session()
- cli.py: new_session() accepts title=None, sets title via SessionDB
- gateway/run.py: _handle_reset_command() parses title, sets on new entry
- gateway/session.py: reset_session() accepts optional display_name
- tests: add test_new_session_with_title, test_reset_command_with_title,
  test_new_command_in_help_output

All 36 affected tests pass.

hermes_cli/commands.py

@@ -64,7 +64,7 @@ class CommandDef:
 COMMAND_REGISTRY: list[CommandDef] = [
     # Session
     CommandDef("new", "Start a new session (fresh session ID + history)", "Session",
-               aliases=("reset",)),
+               aliases=("reset",), args_hint="[name]"),
     CommandDef("clear", "Clear screen and start a new session", "Session",
                cli_only=True),
     CommandDef("redraw", "Force a full UI repaint (recovers from terminal drift)", "Session",