Context compaction summaries were always produced in English regardless
of the conversation language, which injected English context into
non-English conversations and muddied the continuation experience.
Adds a one-sentence instruction to the shared `_summarizer_preamble`
used by both the initial-compaction and iterative-update prompt paths.
Placing it in the preamble (rather than adding it separately to each
prompt) means both code paths stay in sync with one edit.
Ported from anomalyco/opencode#20581. The original PR (#4670) landed
before main's prompt templates were refactored to share the
`_summarizer_preamble` and `_template_sections` blocks, so the
cherry-pick conflicted on the now-obsolete inline sections; re-applied
the essential one-line change on top of the current structure.
Verified: 48/48 existing compressor tests pass.
13294c2d18