CLI: decorate runtime errors with call-stack trace

format_error_with_trace was already implemented but the CLI was
discarding the trace by printing only the bare error string. Wire
it into the two top-level execute() sites so users see WHERE the
error fired, not just what.

## Before
    Error: Index out of bounds: xs[99] (length 3). ...

## After
    Error: Index out of bounds: xs[99] (length 3). ...
      at inner (6:11)
      at main (8:1)

This composes with all the other error improvements:
- name+length+hint at the point of failure (this commit's wiring
  makes that visible)
- did-you-mean for undefined variables
- reserved-word and `if x = 5` hints from the parser
- cross-container hints (arr_get on dict → suggests dict_get)

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

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: RandomCoder-lab

omnimcode-cli/src/main.rs

@@ -1079,7 +1079,12 @@ fn execute_program(source: &str) -> Result<(), String> {
             }
         }
     }
-    interpreter.execute(statements)?;
+    // Decorate runtime errors with the call-stack trace so users see
+    // WHERE the error happened, not just what. format_error_with_trace
+    // is a no-op when the message already has trace lines.
+    interpreter.execute(statements).map_err(|e| {
+        interpreter.format_error_with_trace(&e)
+    })?;
 
     Ok(())
 }
@@ -1229,7 +1234,7 @@ fn repl_execute(
     }
 
     if let Err(e) = interp.execute(statements) {
-        eprintln!("Error: {}", e);
+        eprintln!("Error: {}", interp.format_error_with_trace(&e));
     }
 }