fix(llm): compact on maximum image errors - #183

A provider's maximum image-count error stops the run instead of enabling context compaction. This change recognizes that explicit refusal through the existing compaction path. It preserves rate-limit exclusions and existing retry limits. The classifier regression fails before this change and passes afterward. Existing overflow tests and all required typechecks pass.

Summary by cubic

Classifies provider errors about exceeding the maximum image count as context overflow so the run compacts instead of stopping. Previously the error was treated as a hard failure; now it follows the same compaction path as token-limit errors, while rate-limit variants remain excluded.

^{Written for commit dabf277. Summary will update on new commits.}