Add --halt-on-error flag for safer scripting

This flag causes Scryer Prolog to terminate with exit code 1 when
encountering errors, instead of dropping into the REPL. This makes
it much safer to use Scryer in scripting contexts, CI/CD pipelines,
and automated testing.

The flag affects error handling in:
- Goal execution (via -g flag)
- File loading/consultation
- Initialization failures
- Exception handling in the REPL

Usage:
  scryer-prolog --halt-on-error my_file.pl
  scryer-prolog --halt-on-error -g "my_goal"

Resolves: #3146

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: J.J.'s Robot <jjtolton@gmail.com>

Author: jjtolton

src/machine/args.rs

@@ -4,13 +4,15 @@ use std::env;
 #[derive(Debug)]
 pub struct MachineArgs {
     pub add_history: bool,
+    pub halt_on_error: bool,
 }
 
 impl MachineArgs {
     pub fn new() -> Self {
         let args: BTreeSet<String> = env::args().collect();
         Self {
             add_history: !args.contains("--no-add-history"),
+            halt_on_error: args.contains("--halt-on-error"),
         }
     }
 }

src/toplevel.pl

@@ -79,6 +79,7 @@
         ;   member(Arg0, ["-g", "--goal"]) -> gather_goal(g, Args, Goals0)
         ;   member(Arg0, ["-f"]) -> disable_init_file
         ;   member(Arg0, ["--no-add-history"]) -> ignore_machine_arg
+        ;   member(Arg0, ["--halt-on-error"]) -> ignore_machine_arg
         ),
         !,
         delegate_task(Args, Goals0)
@@ -100,6 +101,8 @@
     write('Fast startup. Do not load initialization file (~/.scryerrc)'), nl,
     write('   --no-add-history       '),
     write('Prevent adding input to history file (~/.scryer_history)'), nl,
+    write('   --halt-on-error        '),
+    write('Terminate with exit code 1 on errors instead of entering REPL'), nl,
     % write('                        '),
     halt.
 
@@ -154,19 +157,31 @@
               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,
+                  (   halt_on_error_enabled ->
+                      halt(1)
+                  ;   true
+                  )
               )
         ) -> true
     ;   write('% Warning: initialization failed for: '),
-        write_term(Goal, [variable_names(VNs),double_quotes(DQ)]), nl
+        write_term(Goal, [variable_names(VNs),double_quotes(DQ)]), nl,
+        (   halt_on_error_enabled ->
+            halt(1)
+        ;   true
+        )
     ),
     run_goals(Goals).
 run_goals([c(Mod)|Goals]) :- !,
-    (   catch(consult(Mod), E, print_exception(E)) ->
+    (   catch(consult(Mod), E, (print_exception(E), (halt_on_error_enabled -> halt(1) ; true))) ->
         true
     ;   write('% Warning: initialization failed for: '),
         double_quotes_option(DQ),
-        write_term(consult(Mod), [double_quotes(DQ)]), nl
+        write_term(consult(Mod), [double_quotes(DQ)]), nl,
+        (   halt_on_error_enabled ->
+            halt(1)
+        ;   true
+        )
     ),
     run_goals(Goals).
 run_goals([Goal|_]) :-
@@ -542,6 +557,10 @@
     gather_equations(Pairs, OrigVarList, Goals0)
     ).
 
+halt_on_error_enabled :-
+    raw_argv(Args),
+    member("--halt-on-error", Args).
+
 print_exception(E) :-
     (  E == error('$interrupt_thrown', repl) -> nl % print the
                                                    % exception on a
@@ -550,7 +569,11 @@
     ;  true
     ),
     loader:write_error(E),
-    nl.
+    nl,
+    (  halt_on_error_enabled ->
+       halt(1)
+    ;  true
+    ).
 
 print_exception_with_check(E) :-
     (  E = error(_, _:_) -> true % if the error source contains a line