Replace halt flags with -t custom toplevel option

- Remove --halt-on-error and --always-halt flags and environment variables
- Add -t FLAG to specify custom toplevel (arity 0 predicate)
- Default toplevel is 'repl' if -t is not specified
- Using `-t halt` achieves original goal of guaranteed termination
- Custom toplevels enable flexible exit strategies (e.g., server mode)
- Update help text to document -t flag
- Remove halt_on_error and always_halt from MachineArgs struct

This is more elegant and flexible than the previous approach, allowing
users to define their own toplevel behavior including halt, custom REPLs,
or server loops.

Examples:
  scryer-prolog -t halt program.pl    # Exits after execution
  scryer-prolog -t my_repl program.pl # Custom REPL
  scryer-prolog program.pl            # Default REPL

Co-Authored-By: J.J.'s Robot <noreply@example.com>

Author: jjtolton

src/toplevel.pl

@@ -16,6 +16,7 @@
 
 :- dynamic(disabled_init_file/0).
 :- dynamic(started/0).
+:- dynamic(custom_toplevel/1).
 
 load_scryerrc :-
     (  '$home_directory'(HomeDir) ->
@@ -53,7 +54,13 @@
     ;   true
     ),
     (\+ disabled_init_file -> load_scryerrc ; true),
-    repl.
+    start_toplevel.
+
+start_toplevel :-
+    (   custom_toplevel(Goal) ->
+        user:call(Goal)
+    ;   repl
+    ).
 
 args_consults_goals([], [], []).
 args_consults_goals([Arg|Args], Consults, Goals) :-
@@ -71,12 +78,13 @@
     args_consults_goals(Goals1, Consults, Goals),
     run_goals(Consults),
     run_goals(Goals),
-    repl.
+    start_toplevel.
 
 delegate_task([Arg0|Args], Goals0) :-
     (   (   member(Arg0, ["-h", "--help"]) -> print_help
         ;   member(Arg0, ["-v", "--version"]) -> print_version
         ;   member(Arg0, ["-g", "--goal"]) -> gather_goal(g, Args, Goals0)
+        ;   member(Arg0, ["-t"]) -> gather_toplevel(Args, Goals0)
         ;   member(Arg0, ["-f"]) -> disable_init_file
         ;   member(Arg0, ["--no-add-history"]) -> ignore_machine_arg
         ),
@@ -96,6 +104,8 @@
     write('Print version information and exit'), nl,
     write('   -g, --goal GOAL        '),
     write('Run the query GOAL'), nl,
+    write('   -t GOAL                '),
+    write('Use GOAL as custom toplevel (arity 0 predicate)'), nl,
     write('   -f                     '),
     write('Fast startup. Do not load initialization file (~/.scryerrc)'), nl,
     write('   --no-add-history       '),
@@ -117,6 +127,17 @@
     Gs =.. [Type, Gs1],
     delegate_task(Args, [Gs|Goals]).
 
+gather_toplevel(Args0, Goals0) :-
+    length(Args0, N),
+    (   N < 1 -> print_help, halt
+    ;   true
+    ),
+    [TopLevel|Args] = Args0,
+    atom_chars(Goal, TopLevel),
+    retractall(custom_toplevel(_)),
+    asserta(custom_toplevel(Goal)),
+    delegate_task(Args, Goals0).
+
 disable_init_file :-
     asserta('disabled_init_file').
 
@@ -154,7 +175,7 @@
               Exception,
               (   write_term(Goal, [variable_names(VNs),double_quotes(DQ)]),
                   write(' causes: '),
-                  write_term(Exception, [double_quotes(DQ)]), nl % halt?
+                  write_term(Exception, [double_quotes(DQ)]), nl
               )
         ) -> true
     ;   write('% Warning: initialization failed for: '),