bs: persist REPL command history across sessions

Load and save readline history in $HOME/.bs_explorer_history (capped at
1000 entries), so up-arrow recalls commands from previous sessions.
Saved on exit/quit, EOF and Ctrl-C.

bs/main.c

@@ -16,10 +16,34 @@
 static jtag_core *jc = NULL;
 static script_ctx *sctx = NULL;
 
+#define HISTORY_MAX_ENTRIES 1000
+static char history_file[1024] = "";
+
+/* Persist the REPL command history across sessions in
+ * $HOME/.bs_explorer_history (skipped if HOME is unset). */
+static void init_history(void)
+{
+    const char *home = getenv("HOME");
+    if (home && *home) {
+        snprintf(history_file, sizeof(history_file), "%s/.bs_explorer_history", home);
+        using_history();
+        read_history(history_file);
+    }
+}
+
+static void save_history(void)
+{
+    if (history_file[0]) {
+        write_history(history_file);
+        history_truncate_file(history_file, HISTORY_MAX_ENTRIES);
+    }
+}
+
 static void handle_sigint(int sig)
 {
     (void)sig;
     fputs("\n", stdout);
+    save_history();
     bsexp_deinit(jc, sctx);
     exit(0);
 }
@@ -91,6 +115,7 @@ int main(void)
     signal(SIGINT, handle_sigint);
     rl_attempted_completion_function = bs_completion;
     rl_readline_name = "bs_explorer";
+    init_history();
 
     print_banner();
 
@@ -116,6 +141,7 @@ int main(void)
         fputs("\n", stdout);
     }
 
+    save_history();
     bsexp_deinit(jc, sctx);
     return 0;
 }