Make gpt-6.1-sol the default compactor model

The compactor uses gpt-6.1-sol at high effort unless config.json names
another model. In the probe run it needed 1.10 calls per model line, and
2 of 388 lines went over 512 bytes, where gpt-6-luna needed 3.3 calls
per line and kept half of its lines over the limit after five tries. It
also credited quoted tool text to the tool, where gpt-6-luna credited it
to the user. 425 calls for 300 messages moved the weekly quota window by
about one point.

Issue: b14d537