`swaig-test`: surface backtrace on load errors

[cassieemb]

Summary

When loading an agent file raises, the CLI prints only the error class/message with no location, so users can't find the failing line.

Where

bin/swaig-test (the rescue around file loading; grep Error loading).

Directive

Include e.backtrace (at least under --verbose; ideally the first few frames always) so the offending file:line is shown.

Acceptance

• a deliberate error in a loaded file prints its file:line

Verify

Point the CLI at a file that raises on load; confirm the location appears.