Add support for showing variables

[bendavis78]

Problem:

get_issue_details returns stack frames with file, line, function, and source code, but not local variable values — even when Sentry has captured them. Without variable context, the assistant can't actually diagnose the issue — it has to punt back to the human to check the UI. That defeats the purpose of the MCP integration.

I ran into this today. We had an EncodeError: Object of type str is not JSON serializable in a Celery/Kombu stack. In the Sentry web UI, I can see the value of o at the crash site (json/encoder.py:180) — it was a SimpleLazyObject. That's the whole answer to "what was being encoded?" But the MCP tool only gave me frame metadata, so I had to leave the AI workflow and go look at the UI myself.

Proposed solution:

Rather than bloating every get_issue_details response with variables from every frame, add a separate tool like get_frame_variables(issue_id, event_id, frame_index) that returns the local variables for a specific stack frame. This keeps the common case token-efficient while still letting the assistant drill into the frame that matters.

Another option to save on # of requests might be to include variable data only for the top/bottom frame of each exception by default, since that's where the most useful context usually lives.